I always have to get it through my head that Velvet is actually a gilf now.



Albeit as Sgt.Bright says even in her base form she looks pretty nice and in shape (she has been depicted as a bit of an adventure/thrill junkie anyway).



Personally I blame it on the possibility that ponies (here in this pic of course transformed into a sexy human) can live pretty long, considering their natural magic capabilities and all that, so most of them if not all would outlive a human by quite a bit.



That means staying younger and in shape for longer on the outside as well.



Im not really talking about them growing up more slowly compared to humans (like dragons), but rather the extension of their prime-time by more then a small margin.



What I'm proposing is this:

They grow from child to adult much the same as humans, but stay longer in their prime and become older/degenerate later than humans after that time.



Best example would be Granny Smith. According to the founder story of Ponyville, she has to be around 100 years old, yet can perform physical feets at such an age which would be utterly impossible for a human to replicate.



Hence, even if Velvet were to hit 40-45, she may still look and feel like 25-30. That would also extend the time span for them beeing able to reproduce… hmm Velvet might be thinking about getting her stallion to give Twilight another little brother? :D