Let's not make this like every family has the uncles and aunts younger that their nieces and nephews. It is uncommon, because most children do not have children until after their parents are done having children, and yes I get with the rise in teen pregnancy, along with women having children later and later, it is probably becoming more common, but to make it like a normal everyday thing, is just not correct. Of course, Once is in a class all its own as we have characters who are (in mind, not in body) hundreds of years older that their love interests (looking at you Rumple and Belle, and Hook and Emma), but this is somewhat common in fantasy/sci fi where people can somehow stop the aging process. So in conclusion, Once has very weird family dynamics, which is part of the fun of the show.
Well, my father's oldest sister is a few months younger than my maternal grandmother, and his youngest brother is my mom's age. The aforementioned aunt has a daughter (my cousin) who is just a couple of years younger than my mother, and I myself am just a few years older than my eldest cousin once removed. So I missed such a situation in my family just narrowly and can confirm it is indeed very possible. Perhaps not common, but quite normal.
So yeah, I've got an aunt and uncle with a 25 year age gap without the Dark Curse; in the Onceverse it can very well be more plausible!
my friend is only 3 years older than her nephew, this friend is also a step-great-aunt, because he niece's half-sister was a teen mum, yeah step-great-aunt, my friend is only 20. She has another nephew from another 1/2 sister, who is 4 yers her senior. And I have family friends who are older than their aunts, likemy mum's friend who's aunt is one year younger so they always acted more like cousins due to their age. if you and a sibling (or 1/2 sibling) have a significatnt age gap, being youger than your niece/nephew is very plausible, although not too common, i've known my friend for 15 years now, so this doesn't phase me.
but i find if you're from a mixed family (like with 1/2 and step siblings) it's more common because you're more likely to have an age gap with 1/2 siblings opposed to full, although it is possible there too, like Bill Murray's brother (full sibling) is 15 years his senior. So even though age gaps are less uncommon amongst 1/2 siblings it can still happen with full.