Hm, I think there needs to be a clear differentiation between Rumplestiltskin the Dark One and the ‘Papa’ one. I think when we say ‘Rumple is 300 years old’, we’re talking about the Dark One. His body isn’t that of the ‘Papa’ any more, as we see, it sort of changes magically into this Crocodile. Therefore if his curse is broken, I’m pretty sure his body would come back to that of ‘Papa’ age and appearance. I mean, look at Mr Gold and he’s doing just fine and that’s supposed to be a land without magic.
[same with Blue Fairy, I mean, she must be 300 years old as well, if not more!]