Me and DH were talking about this last night and he believes the opposite is true.
I think that for girls it's acceptable, normal even, for them to play with baby dolls and have 'childish' interests for longer than boys are 'allowed' to.
I'm basing this purely on observing DS (5) and his friends in his year 1 class. DS loves the Octonauts and a few other TV shows, which obviously make up a lot of what he talks about. Lately he's been telling me that his friends think he's babyish, that he should be playing Minecraft and watching the Avengers films instead (he's not allowed to do either just yet) which is what they all seem to be doing. We walk to school with a few of his classmates and I've heard it firsthand too. He obviously feels as a result that there's a pressure there to be more grown up, which is sad.
In contrast, the girls in his class are all rainbows and baby dolls. There doesn't seem to be that pressure for them. DH pointed out that in few short years, there will be immense pressure for them to appear as grown up as possible, along with all the other pressures that come with being a teenage girl. But it's the same for boys at that age too, to a certain extent, isn't it?
AIBU to think boys experience it earlier and wonder why that is?