No matter what your criteria is there are always exceptions and I've met them much younger than I would otherwise be embarrassed (but legal tender..lets not get it twisted..lol) and way older than I normally would have otherwise. Generally, I think its about a few things.
Men have to be physically attracted to the woman. I have made exceptions because the woman was 'beautiful' on the inside. My 'spiritual' wife is Nina Simone. Had I lived in her era, I'd have wifeyed her up quick. I actually have a thing for dark skin women. She isn't your 'classic' beauty but her soul resonates. I'd have married her at pretty much any age she was...lol.
One of the problems for many men, including myself these days is the sistas around our age is so damaged emotionally (I'm not blaming them all, some brotha or two did some gully ish emotionally on some) that you can't even date much less marry. I think the same in reverse for women I'd imagine.
There is something to be said about 'catching' a woman young enough that she hasn't been jaded and too cynical yet. My preference are black American women but its hard as hell to find quality over a certain age. American sisters are what I know best, what I grew up with, dated, befriended as homegirls, the whole 9. The term 'quality' is subjective, I may not be able to fully define it but I know it when I see it. And to be fair, I'm sure sistas can say the same about brothas in general.
I've seen that quality in foreign sisters but at the end of the day the cultural divide and beliefs (seeing white supremacy for what it is), makes many if not damn near all of them a no go for me. Unless you can mold them. (stop rolling your eyes sistas, yes, I said 'mold' haha). Canadian and some British sistas come closests. Especially Canadian sisters.