One thing I've seen is, in older girl younger guy, they deliberately drop the girl's mental age to put them on equal footing. The pairing is so rarely well executed. I could probably find better doujins than manga for it.
Its even more rare in visual novels, which kinda ticked me off since I considered it to be "broad" in genres.
I tend to keep an open mind regarding age gap, so I try to go in neutral for both. Though 90% times I end up liking older girl romance more, despite its shortcomings.