Peer pressure is real. Kids get social media accounts way too early because it's difficult to justify holding off when all of their classmates have them. It causes actual social issues for kids when they are the only one without something. They get bullied etc, so parents are effectively forced to accede. Making it illegal gives parents a reason to say no, which might slow down the uptake.
I'd really like to see a teenager who'd say "yes parent, I'll not use Instagram because you told me not to". People who pushed this law are so senile they frankly forgot what teenagers even are.
What? You think every single teenager universally disobeys their parents? I know for a fact this isn't true. There exist responsible teenagers. Brides, even if a teenager is disobedient, the placement of boundaries changes their behaviour.
For something so menial like social media? That's equivalent of tying your shoelaces in their eyes. I'm willing to bet you real money that this would be <1% of teenagers. If anything, I wouldn't be surprised if this would have the opposite effect.