You are plenty young enough don't let that bother you. First thing you need to do is go to a recruiter and test the ASVAB. This will tell you what you qualify for and you can decide what you want to be as well.
As for the services,,,,, man that is tough...
Air Force has by far the best quality of life but depending on your MOS (Specialty) your advancement could be gridlocked for awhile. Also heard they are letting out a lot of people so that could change things. Talk to a recruiter.
Marines- Not as bad as you might think. They have an air wing that doesn't do the grunt thing and you get to carry a BFG at times. The most disciplined out of all the services but then again, ya gotta be.
Army- they have an airwing as well and the coolest of all the helicopters. Odds are if you aren't a medic or an OB (OFFICE BITCH) you are going to the front line.
Navy-near and dear to my heart. They have it all, surface, air, subsurface, SPECOPS. Pick your poison. Deployments go by fast and you will see some awesome things.
All services have enlisted aircrew as well. Highly reccomend you attempt to get something in the aviation ratings and fly. Much more fun than just sitting on the deck somewhere waiting for the plane to come back. Extra work but often times worth it.
Here is a link to the Navy Times... http://www.navytimes.com/channel.php?GQID=NAVYPAPER
Didn't look through it so can't tell ya what to expect inside. Ya can do the same for the other services.
Don't forget about our Knee High Navy, the coasties. Awesom Org. as well.
Either way you go, you get out of it what you put into it.