Looking younger is attractive for women because straight men value it (most likely for biological reasons related to fertility), and women in general have features that make them look younger and softer than men. (You’ll notice women tend to have less body hair in general.) Now, while the timing of puberty and so on means that this doesn’t entirely make sense for armpit hair, it isn’t one of those preferences that has to be unified so long as other traits of sexual maturity are present.

For straight women seeking men, youth is still a benefit if it doesn’t get in the way of other matters, and there is some debate about the influence of widespread use of hormonal birth control on sexual preferences, in addition to the usual debate about social formation of preferences.

LGBT individuals, of course, have no reason to be in any particular preference grouping on this matter.