While everyone will talk at length about the obvious reasons, one important one to consider is physical biology.

A girl flashing her boobs to a teenage boy isn't a super huge threat, because even at that age he is likely able to overpower her. That's just how our dimorphism works. Its why almost all sexual assaults from women involve emotions and social threats, or weapons. So if he is at an age where he can enjoy it, its instinctual to see it as harmless fun because we also don't recognize her as a threat to him.

The flipside being that the adult male doing the same can easily overpower the younger girl. He is an actual immediate physical threat, and we all recognize that on a subconscious level. His intents might be equally harmless, but he has the capability to be a problem and that makes it seem far worse.

Of course this isn't the case everytime and there is always a bunch of contexts, but like all double standards it starts on instinctual reactions and then we backtrace a justification to guess why we feel that way.

There is also the difference in our brains, which is that I have seen women often and surprisingly easily flash their tits with zero intent to sleep with a person while I can't think of a man that would flash the rod without being at least willing to fuck them. You figure that one out, because its too dumb for me.