Most kids probably don't wear them once they get to near teens but I'll be making sure my 12 year old gets one now after his mate had a bad accident earlier. All the kids were riding their bikes in the street and one 12 year old came off (freak accident, hit a bump in the road whilst racing) flew two car lengths and landed 6 feet away from his bike. The screaming we heard was unbelievable!
The upshot is after one ambulance, one paramedic car, the air ambulance and two police cars attended the poor little sod is now in the Royal London Hospital with a broken shoulder and a fractured skull doped up to the eyeballs on morphine and having to be carefully monitored because of his ear bleeding...
He was brave though and stayed conscious and talking the whole time. He was more worried about his ankle hurting (nothing wrong with that though, just skinned it).
Having his school uniform cut off didn't please his mum too much though!
So...make your kids wear their helmets!