Comment what age you think they would evolve

I would say 13

To evolve into a Sylveon-human, you would need to be 69 years old and mentally insane to evolve. (No offense)

Actually leveling up isn't related to age, but combat experience.
Although lower levels would usually be younger people and higher levels would be older, just because younger people wouldn't have had to time to level up that quickly.
They are still unrelated and it would be possible for a really young person to evolve, or a really old person to never evolve.