IMHO, if you're going to join the army then its better to do it fairly young. But before I go any further, I want to make one thing clear: GRADUATE HIGHSCHOOL!!!
Now, on to my point...
The military is a good place to build maturity, make a bit of coin, and decide what you want to do with your life in the long term. The way I see it, there are plenty of kids who go to college/university right out of high school with NO idea what they want out of life. They go because their parents want them to go. They choose courses that they think they might like. But they really they just end up pissing mommy's and daddy's money away at frat parties simply because they don't have the maturity or discipline to excel.
If you've always been interested in the army, and you have a strong sense of duty towards your country, the army is a great place to sort yourself out. The army has a great way of teaching you that hardship is NOT forcing yourself to study. Hardship is actually more like forcing yourself to not quit when you've been digging trenches, not sleeping, and being cold and wet for days on end. It really puts things in perspective. ;D
No one says that you need to make a career out of the army. If your place in life is to be a cop, teacher, or any other valued member of our society and the army has helped you in your goals, then the army is maintaining its own goal of a safe, stable, and prosperous country.
Take a friend of mine as an example: He graduated high school and went to college, but after a year or so he decided he wasn't ready for school. He thinks the Infantry would be pretty cool, signs up, and off to basic he goes. Now, his contract is up in a few months and has accomplished the following:
1) Provided outstanding military service to his country.
2) Payed off previous student loans.
3) Saved enough money to pay for further post-secondary education on release (accepted to Carleton U in Ottawa for September).
4) Gained the maturity and discipline from military life he needed to reach his full potential as a civilian university student.
5) He's under 25 years old, and will enter into the civilian workforce at a comparable age to his peers.
I'm not saying "Join the army to get an education". The LAST thing I want is to work with guys who's heart isn't in it. I'm saying that the experience of being a good, solid troop can help to get your life on track.
Good luck!!!