I'm pretty sure you have to voluntarily choose to be part of a medical trial which means age of consent is 16 - of course people under 15 haven't been part of clinical trials - they test these on older people first and once deemed safe they give to the younger population.

Why do 12 year olds need the vaccine anyhow? I won't be giving it to my DD - If she wants it she can choose to do it herself before she gets married etc.