You have to take everything with a grain of salt when it comes to fantasy, sci-fi, and fairytales. Such is the suspension of disbelief when magic is the explanation for long life. Plus, when many of these original fairy tales were written, age differences were much more acceptable amongst readers, since women as young as 14 often married men twice their age. With respect to Once, I’m not overly concerned about the age differences, since as has already been pointed out, the men in question (Rumple, Hook, and Neal) are frozen at the age they were when their biological aging came to a halt. The same is true of Snow White and David who are supposed to be an entire generation older than Emma but are actually her peers due to time being frozen during the curse.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy