The more I watch Tate no Yuusha, the more it's obvious that the only character to which the rule of "growing with levels" actually applies is Raphtalia and Raphtalia only. Atla, Fohl, Kiel and so many other demi-humans already reached high levels by the season 3 and still look like little kids, even though by lvl 18 Raphtalia looked like a teen (episode 3 of the first season), and even older when she was lvl 24 (episode 4 of the first season). Atla is level 25 while Fohl is level 47 in episode 12 of season 3, yet they both look like kids. I don't remember Kiel's level, but it's definitely high, considering she was participating in the class-up ceremony at the Dragon Hourglass. I heard that the novel had some retcon to how demi-humans age and that anime took that retcon into account, but it's still pretty inconsistent and vague how it works. Even with further explanations in the replies to the link I gave, it's not really something that feels like it was thought out by the author well enough, because if there truly are different rates of growth per animal species, they should have been listed out by the author of the novel themselves with level thresholds, blessings and curses that can affect the rate etc.