I always figured it the rate of ageing was different for each half-manakete. I mean, if you think about the whole science aspect of it, you never really get a perfect 50-50 split when it comes to inheritance, right?
So Sophia got the really-slow-ageing but not the ability to transform, and Nn got the ability to transform but ended up with a slightly more normal growth. She still probably ages slower then everyone else--in all honesty, she doesn't look like she's the same age as the other children characters--but not nearly as much as Sophia.
Of course, that doesn't solve the "when does the ageing slow down" question at all.
The main problem for actually answering this question is the incredibly small group we have to compare. There's no more than four half-manakete's in the entire series, and only two potential quarter-manaketes, Mark (With Nn as the mother) and Roy (if you think Ninian is Roy's mother).
Presumably, Ninian and Nils are the same age, more or less, and yet Nils looks a fair bit younger than Ninian. Nn seems to age relatively fast in comparison, looking to be about the same age as Nils, despite being less than 1/20th of his age. And then there's Sofia, who looks to be the same age as Ninian, despite being much, much younger.
The only theory I can suggest is that different subspecies of manakete (fire, ice, divine etc etc) grant different traits to half-manaketes. Presuming that Nono is a Divine Dragon, and that Sofia's manakete parent was also a Divine Dragon, then it would appear that half-manaketes of Divine Dragon ancestry age significantly faster than half-manaketes from an Ice Dragon.
Edited by Onestep, 08 August 2012 - 06:13 PM.