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I need an explanation for how one could be immortal and still age. If one was truly immortal one would remain an infant forever. How could this be explained?
I'm working on a character who is supposedly immortal, but is also in his later years (an old man). How would that work? Keep in mind he's been immortal since his birth and was not bestowed this attribute by anybody during any point in his life.
Eternal youth and immortality are two completely different superpowers.
Eternal youth bestowed at the moment of birth is a curse. While the person could learn things, the lack of motor control and the lack of pruning in the brain would be permanent (unless you change the specifics of the curse). Eternal youth bestowed at, say, age 20, is something most people would consider a blessing (with caveats).
Someone with eternal youth can die like any 20 year old (or whatever age the body is) can die. The body's functions and ability to heal will stay consistent with a 20 year old's but injury and infection kill young people all the time. Someone with eternal youth won't get age-related disease and will have the healing power young people have, but even 20 year olds in the real world get Type II diabetes or heart disease or have strokes because of poor nutrition or chronic drug use, etc.
Immortality has different definitions depending on the author. In some cases, you simply can't die. Look at Captain Jack Harkness from Doctor Who and Torchwood. He can be encased in cement for 1000 years, or exploded into a million pieces, but he always comes back. The Doctor is immortal in the sense that he can in fact die but it takes an awful lot, since his species is long lived and can regenerate.
In other cases beings are called immortal even if they can die from injury or disease because they don't die from aging and generally they have more time to heal from things that would have already killed a regular human.
In other words, you as the author get to choose. If you want immortality to go hand and hand with eternal youth and both are bestowed at birth, well then you end up with a society filled with intelligent worldly infants who can't hold their heads up or articulate words very well (some real life adults are like that, see My Left Foot, and adaptive equipment might become very popular).
Immortal does not equal ageless. That said, for convenience's sake, I'll abuse the word "age" in the same way you were below.
I've seen a number of answers to this question. Most of these concepts can be combined:
The character ages at a slower, possibly much slower, rate.
The character stops aging when they become immortal. (Not your case, but I'm including it for the sake of completeness, since SE is all about making a comprehensive Q&A site.)
There's a difference between aging and growing. The character grows through a normal maturation process, but they just don't age as we normally think of it. (So they keep the healing power of a baby, rather than slowing down to the healing power of a teenager.)
The character's aging slows down as they get older.
The character ages as normal, they just don't die from it. This option results in extremely serious decrepitude after the character is effectively hundreds of years old.
At any point in the character's aging process, they can reset to a younger age
At some point in their lifecycle, the character just dies and immediately starts over. I've seen this method done with both starting over at birth and starting over at conception.
Actually, the movie Star Trek: Insurrection provides interesting insight into this very question.
In the movie, the Baku are a people who live in the "briar patch," a patch of space littered with radiation, etc. What it does for them is give them incredibly long lives having basically stopped aging. From the novelization of the movie we read:
Anij unwrapped her arms and leaned forward. "There's an unusual metaphasic radiation coming from the planet's rings. It continuously regenerates our genetic structure. You must have noticed the effects by now."
Picard gave her a small, sheepish smaile. "We've ... just begun to." He took the cup proffered by Artim, and inhaled the steam: it smelled of pomegranates and flowers. He looked up at the boy and said wryly, "I suppose you're seventy-five."
Artim blinked at him, then with charming childlike candor said, "No. I'm twelve."
The adults smiled. "The metaphasic radiation won't begin to affect him until he reaches maturity," Tournel explained. (Source)
The usual requirement for achieving the goal you seek is to declare whatever it is that grants immortality to be ineffective before physical maturity.
Yeah, but what if I want an old man?
You have two ways of thinking about this one:
The first is the Insurrection method, but rather than the immortality triggering at physical maturity, it triggers at a point of old age. The human body has several stages of development. Simply: childhood to puberty (0-12ish), puberty to physical maturity (12ish-25ish), physical maturity to middle-age (25ish-55ish), middle-age to golden years (55ish-75ish), golden years to venerable (75ish-death). There are actually more, very distinct stages, but these will suit. You can pick any one of them and simply stop the aging process when the body goes through it's chemical changes for that stage.
The second is to not really be immortal, but to age really, really slowly. This is kind of the "Elf" solution to the problem. You live thousands of years — but your aging process is linear through that period and you eventually die.
But, none of this is exactly what I want
If those solutions (or another provided by one of our other community members) doesn't suit, then you need to choose. You aren't going to find a scientifically sensible way of describing this because it doesn't exist. You need to come up with a narrative mechanism that explains the situation and allows you to move on with your story....
Rats... I just realized your question is primarily opinion-based. OK! You got lucky! You're a new user, so please take a moment to read up on what primarily opinion-based means. You might also be interested in the difference between asking for an infinite list of things (off-topic) and a finite list of things (on-topic). Cheers!
If by "in his later years" you mean that he has the external appearance of an old man, that's easy: only his innards are immortal, and he hasn't molted in a while. The other answers explain maturing to adulthood.
I have seen this handles one particular way in fiction and mythology, a person is immortal but ages either normally or at a slower pace. But a sufficiently destructive injury (one that would normally be fatal to a normal person) restores the person to their prime as the body repairs the damage. Basically they age and reset over and over again resetting to their prime every time they "die" sometimes the "death" and reset are periodic instead of traumatic, often both.
Alternatively they age to some arbitrary point then age in reverse until they are back in their prime then start aging normally again. Some versions or merlin use this method.
To further other people's points, the main difference here is the difference between growing and aging.
Growing is a "deliberate" (in as far as a natural process can be deliberate) process the body undergoes to get from fertilised-egg to physical maturity. This process stops at around age 17-18, AFAIK. As you may have noticed, 90 year-olds are not giants compared to 18 year-olds.
Aging, on the other hand, is caused by faults in cell replication which causes the body to slowly deteriorate in quality. While others have said your immortals are not necessarily ageless, I would think these faults would have to be eliminated or somehow worked around to facilitate immortality.
So if you follow this logic, then your immortals would be stuck at around young-adult / old teenager stage, not infants.
Though you want someone in their later years, so physically elderly. You could say they still age to a certain degree, and whatever biological mechanism allows for immortality halts the aging at a certain point once it "adapts" - kind of like the way the immune system takes a bit of time to figure out how to stop a disease.
Know how your mental state can affect your physical well-being? At the simpler and easily observable end, stress from work can induce migraines; joy can put an extra spring in your steps; etc. At the extreme end(which may not necessarily be backed by science), despair and hopelessness can kill, and a strong determination to live on can bring the critically injured back from the brink. From there on I'll extrapolate:
Your mental state/age is your primary or even sole determinant of your body's condition
The character's body develops along with his mind as he goes through the usual process of growing up. However, he stays in his physical prime for far longer than non-immortals do. After all, people don't necessarily define their mental self image as busy working people in their 30s and 40s, it's more like something life inflicts on them. The successful career man who has climbed to the top of the ladder is usually middle aged; he may wish he was in his 20s and try to act like it but his body will tend to dispel that illusion. For your character it simply doesn't work that way, as long as he thinks "I'm an adult at the top of the world" he will be at his physical prime.
What would really cause the character to age is world weariness, the mental burdens of life in general piling up. This would be in stark evidence when the people who he's connected and would normally be his peers begin to pass away. Loneliness is a silent killer that saps the elderly in our real world, and I'd think your character would feel it too.
