There are laws that make countries set an age for when companies can collect children’s information without asking their parent/carer first. Social media sites and apps set this as their minimum age so they do not have to ask your parent’s permission to collect your information.
It is not illegal to sign up to social media if you are underage, but it does break the service’s rules. If they become aware you are underage, they may delete your account.
Remember, if a site/app has a minimum age, then it might also contain adverts and things that people share that are suitable for older children or adults. Using this site/app might mean you see things that aren’t suitable for your age group.