Yeah you can, there's nothin stoppin you. But take care not to over-exert yourself. Also why do you want to go to the gym at such a young age, you can do jogging instead. And don't lift dumb-bells because as you grow older, your skin also grows and if your body is already ripped it'll tear itself to accommodate it. Man, it's painful. I got some marks on my body because of that. I'll upload a few pics to show you.