@El_Gato True, but I'd assume they still reach physical maturity within about 20 years. Otherwise they'd be finishing their educations before physically growing up and there'd be a bunch of fillies in the workforce.
@El_Gato I know, I agreed with you! It's also implicit since Starswirl was Clover the Clever's mentor before the princesses ruled Equestria, but was still alive and became friends with Scorpan many years later.
That's one of the reasons I would jump through a portal which leads to Equestria. Just to live for centuries. Not as good as being immortal, but then I could do some research on how to become an Alicorn without the help of Celestia.