agingdnd-5espells
A 280-year-old gnome is reincarnated into a human. Does the formerly-a-gnome-but-now-a-human promptly die of old age? Has the gnome/human just reached maturity? Could the gnome/human instead have a child's body? Could a dead baby be reincarnated into an adult body? Is there official word clarifying this?
In short, what biological age is a reincarnated body?
Yes you can. The spell never makes any specific claims about the age of a creature, only its challenge rating.
If you turn a creature into another kind of creature, the new form can be any kind you choose whose challenge rating is equal to or less than the target’s (or its level, if the target doesn’t have a challenge rating). [PHB 283]
So you could turn yourself into an elf or similarly long-lived creature which is also a level 20 wizard.
If you wanted to turn yourself creature like a dragon with the intent of doing this repeatedly, you need to be sure to note the spell casting abilities of your new form. From the spell description:
"The creature is limited in the actions it can perform by the nature of its new form, and it can’t speak, cast spells, or take any other action that requires hands or speech unless its new form is capable of such actions." [PHB 283]
If you choose a creature that can innately spellcast (or it has hands and can speak), then this isn't a problem.
The text on Timeless Body says:
Timeless Body
At 15th level, your ki sustains you so that you suffer none of the frailty of old age, and you can't be aged magically. You can still die of old age, however. In addition, you no longer need food or water.
As a monk, they simply suffer "none of the frailty of old age," which means even as old people, they can still move like they were in their 20's. This makes sense when you consider the amount of old monks who can do flawless kung fu in media. They usually serve as mentors to the younger folk so that their secret techniques don't die with them.
- From "The Invincible Armour" a 1977 John Liu film.
In conclusion: Timeless Body still lets you age, and you may still die of old age naturally. However, due to your timeless body, you don't get the back pain or blurry vision that comes with senescence.
Best Answer
Reincarnate causes the new body to be an adult body for that race
So, whatever adult means for that race will be what body you get.
Any additional refinement or details on top of that (such as, what precise age the body is) must come from a DM.
Does the human immediately die of old age? - No
It is a body's age that determines when something dies, not the age of its soul. In this case there is a difference between the two.
Jeremy Crawford, lead rules designer for D&D, agrees:
Have they just reached maturity? Could they be a child? Could a dead baby get reincarnated into an adult body?
Regardless of the age of the dead creature, the spell says:
So, it seems that RAW would indicate that a dead baby or child would be reincarnated into an adult body just the same as an elderly creature is.
The soul is always the same age/maturity as it was when it died.
DM can always overrule and have the body be younger if it makes sense
There may be cases where a DM would consider altering the effect slightly to allow for a new baby/child/infant body if that makes sense for their game. And that is perfectly within the rights and power of the DM.
Honestly, the idea of a baby reincarnated in an adult body is pretty terrifying/horrifying in general. At my table, I would houserule that babies come back as babies in an instant unless there was some compelling story/fun-based reason not to.