Vampirism. There are benefits and drawbacks, but one of the benefits is not aging. Great if you get turned in your early twenties, but... what happens if you are turned earlier?
Being turned as an infant would probably not be fun, but how early can we push things? (For simplicity, let's call our hypothetical child-vampire "George".)
- George should be at least somewhat self sufficient and able to perform physical feats "comparable" to an adult, adjusted for size. In particular:
- Can stand upright, walk, run, climb up or down stairs, and generally move about.
- Can eat food, bath, and put on clothing without assistance.
- Can walk/run with comparable endurance (similar length of time, not necessarily similar distance). Can perform physical exertion (proportionally scaled; for example, body-only exercises) with comparable endurance.
- Can lift at least¹ a proportional amount. (For example, at 33", or roughly half the height of an "average" adult in the US, George should be able to lift about an eighth or more¹ what the same "average" adult could lift.)
- Physiological development isn't "necessarily" halted; George just has to look, from the outside, like he is the same age as when he was turned. (So, no bone growth or shape changes, but composition is allowed to change.)
- George should be able to develop mentally to an adult or near-adult state.
- No ongoing magic. (You can hand-wave how George gets to wherever he's physiologically going, but he can't rely on supernatural abilities once he gets there.)
It should be at least somewhat plausible to go fairly early; after all, the world's shortest adult was a mere 21½". How small could you go, especially dealing with child proportions, and still accomplish the above objectives?
(¹ I think that, due to the way the square-cube law works, we might actually expect George's proportional abilities to exceed those of an adult... but I'm not sure.)
For the purposes of this question, ignore the issues that George will have strictly because of a) height or b) apparent "legal" age. People with dwarfism can be self-sufficient by just about any definition. What I'm really asking is what (insurmountable, given the above wiggle-room) problems, if any, George will have from having the body of a child rather than an adult with dwarfism (AwD). In particular, at what point (in development, or rather lack thereof) do the above limitations necessitate a debilitating difference in physical or mental ability when compared to a same-height AwD who definitely does meet the stated objectives?
I'm confident there is an answer, since I'm pretty sure that no amount of hand-waving of "outwardly invisible development" is going to permit a "functional" person by these standards that visually appears to be a newborn infant. (And at the far end, well, if he's turned as an adult, obviously he can function as an adult. This puts both a lower and upper limit on an answer.)