Can't say I've noticed a double standard either. You've mentioned that maturity is a more important indicator of relationship success, and I'd tend to agree with that, but at younger ages (up to early 20s) maturity is closely tied to age because women/girls generally mature at a faster rate than men/boys, so an age gap where the woman is notably older is more likely to have a maturity gap. But as I say, I think you're fighting against a straw man here. There are unjustifiable double standards between men and women in a lot of aspects of relationships, but I don't think age gap is broadly one of them.