I didn't drink anything with caffeine until I was in my 30s and now I'm hopelessly addicted to it!
My son, 11, doesn't drink anything with caffeine and in Coke country that is a chore! As long as I can prevent it I will. Kids will always want to emulate adults, and as adults we have to decide what they will and will not have until it's their choice. If you choose to let them, that's your choice and that's your prerogative.
I just feel that kids plus caffeine plus sugar is a BIG problem. I am convinced that half of the overactive children would be cured if they stopped eating stimulants. Stopping sugar transformed my son. It's one addiction they don't need. I've put my household on an only water to drink regimen. It was not easy but it works for us.