Can girls still wear boys clothes even if hey have started puberty?

Of course! I think it's alright, after all they're just clothes. The only reason why we wear them is because they provide protection and warmth. Yes, boys and girls are in some ways different but even so, should your gender determine what you do or who you are? If you find boys clothes comfortable and like wearing them than wear them. It's your own style. You can't help liking them. It doesn't mean your gay. If you've started puberty then you will of course have to wear a bra but as for shirts and trousers, and maybe even underwear, that's completely up to you as a person. Embrace your style, some people like to express themselves through clothes and just because you wear "boys" clothes it doesn't mean you are a boy or want to change gender. These days, girls are going outside wearing barely any clothes and people are treating this as if it's normal so I don't see it as a bad thing to wear appropriate clothes that you might have bought from the boy's side of the clothes shop.