Your logic is based on the assumption that Tellius and Ylisse have the same laws of nature at work, which isn't stated to be true. It could be that manakete age like normal humans until they get to a certain apparent age. Maybe Nowi is unusually young looking and decides to pretend to be a child. It's also not a good idea to believe that the thoughts of a fan have credibility, especially when the game makes it clear that the contrary is true (the issue of Gaius and Panne adopting Yarne instead of birthing him).
Also, manakete and laguz dragons are different races.