I was just thinking about the age range of the various Dr's we've had regenerations (assuming the same age as their actor) ranging from mid 20's to mid 50's post Regeneration and since its largely a cosmetic choice it got me wondering are there an upper/lower limit on what they'd regenerate into or could for example Capaldi regenerate into an 11 year old boy because he was sick of the achy bones?
I ask because I"m amused by the image of a little kid trying to convince a companion with no idea of regeneration that they're really the old man who was just "killed".
Well, since the justification for a Time Lord changing gender is "Why not?" then you can justify anything. A Time Lord decides to regenerate into a gecko. Why not? A Time Lord decides to regenerate into a Hooloovoo. Why not? A saintly Time Lord randomly regenerates into the Most Evil Time Lord In The Universe. Why not? So why couldn't a Time Lord regenerate into a child?
Now, that's not how I would do it. I happen to prefer the idea that a Time Lord is humanoid — by the "looks exactly like a human" meaning of the term — adult, and never loses his or her fundamental personality. The change in appearance and surface personality don't change the inner being. I wouldn't condone a regeneration into a child form. I go by the notion that the Time Lords grant new graduates of the Academy the gift of regeneration, and as such it is an engineered ability, not a natural one. I don't think they'd engineer it to let you regenerate as a child.
I don't know there could be a case that children will be able to experience or study aspects of a society that being adult would get you excluded from. Though i suppose that'd be more the province of a chameleon arch.
As for the general idea (that being how regeneration works) It's something I vaguely recall things that argue it does have a potentially major effect. On the one hand we have Romana trying on various bodies to find one she liked yet in a novel regeneration is mentioned as changing a time lady's personality so while she still has all her scientific knowledge she's lost her passion for it and needs to force herself to continue working on the vitally important project she was involved in. Then there's another where the doctor is concerned about getting locked into a cycle where he keeps dying and his body keeps altering to try and survive what kills him. Which implies both it is causing changes in response to the cause of death and has limits on what it can do as he knew while he'd change he wouldnt be able to change enough to survive the threat. Not to mention the whole point of the war doctor regeneration was to become a warrior.
In this i can see an argument that a timelord dying of old age would regenerate young to avoid it but i can also see GMs needing a special group to play it sensibly for a prolonged period of time and even then childhood to adulthood is prrobably only 15 years or less but adulthood lasts for centuries. Not to mention the sheer emotional personality of a child would put most time lords off the idea.
I think personally id go with yes but timelords arent likely to do it outside of extremely rare circumstances like a catastrophic regeneration or a chameleon arch to study a specific culture.
In a society where everyone (apparently) can regenerate, everyone would know that becoming a child wouldn't actually mean becoming childlike. A 1,000-year-old Time Lord who regenerates into a child doesn't lose their experience and maturity, and they wouldn't suddenly be excluded from "adult" functions. Thus, there's no childlike perspective to be gained. They're not going to decide to drop everything and go play.
Didn't Titan Comics introduce an incarnation of the Master regenerated as a child, a couple of years ago? I can't remember the backstory, whether it was choice, accident or something that caused it, though.
It just occured to me we also have scions of Gallifrey like River Song who regenerates as a child and you wouldn't want her becoming an adult when she's not mentally equipped to deal with it.
Because she was a child when she regenerated. River Song is not a Time Lord and doesn't follow the usual rules of regeneration. (She is Moffat's irritating pet character who gets to do whatever he wants her to do to satisfy his wish-fulfillment fantasies.) If Time Lords aren't born with the ability to regenerate, then none of them are going to regenerate when they are children.