Is it against the law if you don't wear a bike helmet when you are under 12 years old?

Many states have child helmet laws. They require a helmet until adulthood -- presumably 18.

Don't let a law dictate wearing a helmet! This is America -- you have Freedoms and Rights here! We need more common sense and less laws to save us from ourselves.

Wear your helmet because you are free to do so, you have the right to wear a helmet if you choose. If you wear your helmet they can't stop you. If you choose not to wear a helmet, don't have your guardians whining to us that your brain is scrambled from an accident.

Depends on where you live. Regardless, helmets are a good idea. I ride something like 5000 miles a year and never swing a leg over without one.

It depends, the law differs where you live.
A lot of places is it against the law. But I would check it out,
Even if it's not, it's still a good idea to

Wear one! Simple as that, your skull is like an egg shell. Also, make sure it fits, and you are wearing it right, that is to say, two fingers from the bottom edge of the helmet's front to the bridge of your nose. Also, make sure the strap is snug.