The enforcement side is going to be messy - basically the platforms have to implement age verification gates, and if they don't do a decent job, they face hefty fines. What actually matters is whether Meta and TikTok and the rest will actually bother implementing strict checks or just do the minimum to avoid penalties, which... let's be honest, they probably will.
As for broader impact, this is mostly a France thing for now, though other EU countries are watching and some are considering similar moves. It doesn't automatically apply elsewhere, but when one major country does something like this, it tends to create pressure on other governments to follow, so we might see copycat legislation pop up over the next couple years. The tricky part is that teenagers have gotten pretty resourceful about getting around age restrictions - VPNs, fake IDs uploaded to verification systems, using parents' accounts - so whether this actually keeps kids off social media or just makes it slightly more annoying for them is a whole different question.