I took the America Red Cross babysitting class at twelve and was quite capeable, certified and working. Too bad I couldn't get past beginner swimming certification back then. Took me nine more years to become less afraid of deep water and finally learn.
Depends on your level of maturity. I've been helping my mom with babysitting since I was in second grade, getting more responsibilities as I grew. Now I'm 15 and watch the ankle-biters frequently, even right now.