It is said that they're 16 years of age when they embark for Eorzea at the start of ARR, I heard in an interview somewhere that around the time of mid-end of HW two years have passed. So technically they should be around 18-19 or something like that.
It would be appreciated if they started giving Alphinaud and Alisae some growth spurts in the coming patches. It would be fun to see a small difference everytime until they eventually looked like mature Elezen. But we'll see if SE is brave enough for that haha.