History is full of teenagers and children who were official leaders and who did somewhere between zero percent and one hundred percent of the actual decision making for their groups, depending on various factors.
For example Francis II (19 January 1544-5 December 1560) Dauphin of France, reached his legal majority at the age of 14 in 1558 and became King of France on 10 July 1559 aged 15 years 5 months and 21 days. But he let his mother and the Guises make the decisions.
On the other hand, King Edward VI of England (12 October 1537-6 July 1553) had his age of legal majority set at sixteen, and so never reached it. But he had a considerable influence on the policies of his regents as he grew older.
And the difference is due to their different personalities.
But they were hereditary monarchs. Who would follow the orders of even the bossiest or most competent child or teenager who was not their hereditary monarch?
It is believed that the ancient Germanic and Norse people didn't have any role for minors to inherit the thrones.
In 575 Childebert II became King of the Austrasian Franks age 5 when his father was assassinated. When Clovis died in 511, his four sons became kings, the youngest three aged about 16, 15, and 14. Clovis himself became king in 481 aged 15. Athalaric (516-534) inherited the Ostogothic throne in 526 aged 10.
So there are examples of children inheriting Germanic kingdoms in the 6th century and of teenagers doing so in the 5th century.
According to the Roman History of Cassius Dio, book LXXII, in about AD 170:
Marcus Antoninus remained in Pannonia in order to give audience to the embassies of the barbarians; for many came to him at this time also. Some of them, under the leadership of Battarius, a boy twelve years old, promised an alliance; these received a gift of money and succeeded in restraining Tarbus, a neighbouring chieftain, who had come into Dacia and was demanding money and threatening to make war if he should fail to get it.
Since this was centuries before the earliest known examples of Germanic child monarchs, I don't know if Battarius inherited his leadership or earned it somehow.
in the US Civil War, many thousands of men were commissioned as officers in the various armies and navies. And most of them were men almost old enough to be the fathers of the typical soldiers, who were mostly young men in their twenties. But a few of the officers were very old, and a few of them were very young, but somehow managed to command positions.
Most Union soldiers in the Civil war were members of jointly controlled federal/state volunteer units. Volunteers would be organized into companies and regiments by states and territories. They would be mustered into federal service and then paid and supplied by the Federal government. The men would often elect their officers who would be commissioned by the state or territorial governments. The federal government only commissioned general officers or officers in the rarer federal units.
The Provisional Army of the Confederate States of America was formed in a similar way.
Because the majority of the Civil War units were such volunteer units, most of the officers, especially at first, commanded because their subordinates had agreed to obey them, often by elections. At the beginning of the war the officers and non commissioned officers were usually not imposed on the privates by outside authorities, but chosen by the privates from among their friends and members of their communities.
During the Civil War, most companies had only one first sergeant, six sergeants, and four corporals in their tables of organization, and most or all of them had command duties.
Thus it was much more impressive to be promoted to those ranks, especially corporal, than it is at the present time.
For example, Thomas P. Gere (December 10, 1842-January 8, 1912) was a lieutenant when he earned the Medal of Honor at the Battle of Nashville on December 16, 1864, aged 22 years and 6 days. That's doing pretty good as a young officer, right?
Actually Gere enlisted in the Fifth Minnesota Volunteer Infantry, and was mustered in January 17, 1862, and was promoted to first sergeant and then second lieutenant. His company was stationed at Fort Ridgely, Minnesota, when the Minnesota Sioux uprising started on August 18, 1862, and Captain Marsh and many of the men were killed trying to rescue civilians. This left Lieutenant Gere in command, age 19 years, 8 months, and 8 days. Lt. Timothy Sheehan (age 21) arrived with reinforcements before the Sioux attacked on August 20 and 22.
Lieutenant General Arthur MacArthur Jr. (June 2, 1845-September 5, 1912) enlisted in the 24th Wisconsin Volunteer Infantry and was commissioned a first lieutenant and adjutant August 4, 1862, aged 17 years, 2 months and 2 days. At the Battle of Missionary Ridge, November 25, 1863, thousands of Union soldiers ran up the ridge without orders to drive away the entrenched Rebels at the top. Lt. MacArthur was one of the leaders in that attack, carrying the regimental colors and planting them at the top of the ridge, age 18 years, 5 months, and 23 days, and was awarded the medal of honor. He was promoted to major January 25, 1864 aged 18 years, 7 months, and 23 days, and to lieutenant colonel May 18, 1865, aged 19 years, 11 months, and 16 days. MacArthur was thus called "the boy colonel".
Henry King Burgwyn, Jr. (October 2, 1841-July 1, 1863) also became known as "the boy colonel" (of the Confederacy). He was commissioned lieutenant colonel of the 26th North Carolina in August, 1861, aged 19 years and 10 months, and colonel in August 1862 when still 20.
Uriah Galusha Pennypacker enlisted in the Union Army in 1861 and rose from private to quartermaster sergeant. He recruited a company for the 97 Pennsylvania volunteers and was commissioned captain in August 1861, and Major in October. He was commissioned colonel August 15, 1864, and led a brigade. He was promoted to brigadier general, United States Volunteers, April 28, 1865, and after the war he became a colonel in the regular army in July 1866.
Various sources said that Pennypacker was born June 1, 1844, making him a Major aged 17 years and 4 months, a colonel aged 20 years and 4 months, a brigadier general aged 20 years and 10 months, and a regular army colonel aged 22 years and 1 month. Other sources say he was born on June 1, 1842, thus making him two years older when he achieved those ranks.
Charles Cleveland Dodge (September 16, 1841-November 4, 1910) was commissioned a captain in the 7th New York Volunteer Cavalry in December, 1861, aged 20 years and two months. He was promoted colonel August 14, 1862 aged 20 years, 10 months, and 19 days, and brigadier general USV November 29, 1862, aged 21 years, 2 months, and 13 days. He resigned in June 1863 but led militia against draft rioters in New York City the next month.
Ulrich Dahlgren (April 3, 1842-March 2,1864) was commissioned a captain on May 29, 1862 aged 20 years, 1 month, and 26 days, and a colonel July 24, 1863, aged 21 years, 3 months, and 21 days.
Many accounts of young Rebels claim that T.G. Bean recruited and trained two companies of recruits in 1861 aged 13 before joining the Rebel army 2 years later. But he was actually Thomas Greene Bush, born 19 august 1947, and he was about 16 years and 11 months old when becoming lieutenant and adjutant of the 62nd Alabama regiment in 1863, and 17 years, 8 months, and 15 days old when they surrendered on May 4, 1865.
W. D. Peake (born December 22, 1846) enlisted in Company A, 26 Tennessee Infantry (rebel) and was promoted to sergeant sometime before the war ended in April and May 1865 when he was 18 years and 5 or 6 months old.
Henry Weidensaul was probably born July 1, 1847 and enlisted in the 46th Pennsylvania Infantry in 1861. He was promoted to Corporal May 18, 1863 aged 15 years, 10 months, and 17 days, to sergeant October 1, 1864, aged 17 years, and 3 months, and to first sergeant on July 1, 1865 aged 18 years.
There is a possibility that a James H. Deal became a Rebel lieutenant aged about 13 to 18 but there is no proof of his service.
There is a story that a T.D. Claiborne was born in 1847 and became a captain in the 18th Virginia Infantry in 1861, a Major and a lieutenant colonel in 1863, and died in 1864. However, there also a statement that Captain T.D. Claiborne of the 11th Viginia was born December 25, 1835.
John C. Delany was born April 22, 1848, and enlisted March 5, 1862, in the 107th Pennsylvania Volunteers and apparently promoted to corporal aged fourteen years, eight months, and thirteen days, sergeant sixteen years, five months, and twenty four days, first sergeant sixteen years, eleven months, and seven days, and second lieutenant aged seventeen years, one month and five days, and earned the Medal of Honor aged sixteen years, nine months, and fifteen days.
Gustave Albert Schurmann was born February 4, 1849 and enlisted in the 40th New York Volunteers in 1861. He was apparently promoted to sergeant in 1863 around the time of his 14th birthday.
An E.G. Baxter allegedly enlisted in the 7th Kentucky cavalry (Rebel) aged about 12 and was commissioned an 2nd lieutenant when aged 13, but I have not found any record of that officer.
Musician Charles Edwin King was probably born on April 3, 1849, enlisted in the 49th Pennsylvania Volunteers September 12, 1861 aged 12 year, 5 months, and 9 days, and died of wounds on September 20, 1862, aged 13 years, 5 months, and 17 days. It has been claimed that he was promoted to drum major during his service.
John Lincoln Clem (August 13, 1851-May 13, 1937) of the 22nd Michigan became one of the most famous drummer boys in the Civil War. He was promoted to the little known rank of lance sergeant on September 20, 1863. Thus he became a non commissioned officer aged 12 years, 1 month, and 7 days.
A Charles Carter Hay claimed to have joined the 15th Alabama age 11 and to have been appointed a lieutenant and a captain, but there is no record of such an officer.
Gilbert de Motier, Marquis de Lafayette, (6 September 1757-20 May 1834) was commissioned a Continental major general on 31 July 1777, aged 19 years, 10 months, and 25 days, though he didn't receive large commands suitable for a general until he was 20.
So there are some examples of young leaders who people followed from history.