What is wrong with some elf families having multiple generations alive at the same time? I am descended from a woman who lived to see the birth of a few of her many great great grandchildren, I believe that the Guinness record for most living generations is 5 or 6, and there is a story about a Chinese family in the Tang Dynasty that had 9 generations living in the household.
At the Royal Ark site - India - Pensioners - Delhi - Page 20 The Family of Mughal Padishah Bahadur Shah II (1775-1862) is listed.
Bahadur Shah II's oldest of many sons was H.R.H. Shahzada Muhammad Dara Bakht, Miran Shah, Wali Ahd Bahadur [Mirza Shubu Sahib] 1790-1849, born when his father was 15. His oldest son was Mirza Muhammad Adu Afghan Bakht Bahadur [Mirza Kale Sahib]. b. 1804, educ. privately. m. Mubarak un-nisa (b. 1819; d. after 1862). He d.s.p. after 1863, born when his father was 14 and his grandfather 29.
The oldest grandson to have children was Mirza Muhammad Karim us-Shuja Bahadur. He was born in 1826 when his father was 36 and his grandfather 51 and died after 1849.
Mirza Muhammad Karim us-Shuja Bahadur's sons included Mirza Azim us-Shan Bahadur, born 1846, when his father was 20, his grandfather 56, and his great grandfather was 71, and Mirza Ahmad Shah Bahadur, born 1848 and had a daughter.
Meanwhile Bahadur Shah II had a bunch of other sons, including a 21st son Shahzada Mirza Muhammad Sher Shah Bahadur who was born in 1846, the same year as his oldest grand nephew, and was alive in 1856, and a 22nd son named Mirza Muhammad Sulaiman Shah Gorgani.
So that is an example of a man whose youngest son was born in the same year - or possibly a later one - as his oldest great grandson, and an example of generations in the same family overlapping in age.
Here is an interesting thread:
Here is a list of oldest fathers, although it focuses more on recent cases than historical ones:
I note that an ancestor of mine was born when his father was 50, and that father was 60 when his youngest daughter was born.
Here is a list of women who became pregnant after age 50:
Here is a list of youngest birth mothers:
Here is a list of youngest birth fathers. They are all recent cases and don't include many historical cases.
Here is a list of some royal extremes in British royalty:
At 15, Henry IV is the youngest King to father a child, prior to his accession.
The youngest queen regnant to give birth is Mary II, who gave birth to a stillborn child in 1678, prior to her accession, when she was just 16. The youngest mother to give birth to a monarch was Lady Margaret Beaufort, wife of Edmund Tudor, who was months from turning 14 when she gave birth to Henry VII in 1457.
The oldest parent was Edward I, who fathered his last child, Eleanor, in 1306, when he was 66, almost 67 years old.
The oldest mother was Eleanor of Aquitaine, who gave birth to John, in 1166, when she was 44.
Note that thousands or millions of people are descended from King John, and many thousands are descended from Henry VII, despite the extreme old and young ages of their mothers.
King Edward I, born in 17/18 June 1239, was 62 years, 1 month, and 18 days older than his second youngest child Edmund of Woodstock, 1st Earl of Kent (born 5 August 1301), who has many descendants among modern royalty and nobility.
There are many examples in history where the generations from oldest son to oldest grandson to oldest great grandson and so on have quite low average generation lengths and also many examples, usually from youngest son to youngest grandson to youngest great grandson, etc. have high average generation lengths.
If your elves live for about 250 years, you could make the average elf generation about 50 years, so that an average elf dies about when his/her fifth generation of descendants are born.
If you want three living generations with 250 year lifespans, you could make the average generation about 75 years and elf great grandchildren will be born about 25 years before the great grandparent dies.
Or you could make the average generation 100 years and average elves will died when their grandchildren are 50, and elves would only live to see great grandchildren when their generations are shorter than average or they live to be 300, 1.2 times the average lifespan.
It takes simple mathematics for you to combine elf lifespans and elf generations lengths to give the average elf family the number of living generations living at the same time that you desire. But some elf families will be different from the average and other elves may be favorably or unfavorably impressed with the differences.