Technically it isn't incorrect; originally the term 'evolution' simply referred to any change that occurred over time, and was in fact used to refer to metamorphosis. It was only later, after Darwin appropriated the term to refer to species evolution specifically, that the word took on its more exclusive modern meaning.
Of course, ever since Pokemon used 'evolution' to refer to metamorphosis, it's been kind of a toss-up term among the gaming community.
Back on topic:
It is possible to make reactions/syndromes that take age as a parameter using DFHack. If you don't want to use DFHack, you can use the method suggested already, by adding a self-targeting interaction, used only once in the creature's life, that prevents the transformation and lasts for a long time, until the creature is 'mature'. Admittedly, if you're using that already, you might as well just have the interaction itself transform the creature after a set amount of time. Then you don't even need a reaction.
In either case, it won't be tied to age, the 'timer' will start only after they enter the game. It should also be noted that it may take a while for creatures to use self-targeting interactions, so if you use the immunity method it might be possible for players to perform the reaction before the syndrome actually triggers.