I signed up at 17 but I needed my mother to sign the waiver. After your parent or guardian signs you up, you'll then go into a poolee program where they prep you for bootcamp. Once you graduate high school or become 18 with a GED they will ship to Parris Island or Camp Pendleton for 13 weeks of the hardest basic training the US military has to offer. If you're going to be a Marine you should man up and go infantry then work your way up to Force Recon which is pretty much the closest you'll ever get to being SEAL in that service. Being a support marine sucked for me and you can't really brag about cooking pancakes, packing supplies or counting bullets. I'd tell that to any young service member. If you go Army you need to apply for Ranger/ Delta. Seek to be a badass and you never have to wonder of what could've been like I did. Don't worry about dying or getting killed cause death comes for everybody. Everybody dies kid, but not everybody lives.
I do not think that you are allowed to enlist until your are eighteen years of age. You also have to have a high school diploma or GED. It does not take very long to get started though. If done as a career, you can retire earlier then most.