I joined in '92 at age 17. Minimum commitment is 2yrs + the time you spend in processing, basic training and Advanced Individual Training (AIT). Basic is 8-weeks, the shortest AIT is 4-weeks, and processing generally takes 1-3 weeks.
Basically the least you can join for is 3yrs all inclusive and that is what I did.
The military allows you to select your job prior to enlisting and they base this off of need and your GT score. GT score comes from taking the ASVAB. In the Army there are over 200 different jobs. From military police to infantry to cook to truck driver to nurse to dental assistant to chaplains assistant. A huge range.
When I joined I had a GT score which enabled me to select any job I wanted and I joined up November which usually allows you a better spot of picking your job as jobs fill up fast when the allocation is put out in October each year.
Some jobs also come with bonuses, a signing bonus, money to pay off student loans, choice of duty station, college fund money not to be confused with the GI Bill.
I asked them to run my GT score through the system for all jobs available for a 3yr enlistment and that gave me the maximum Army College Fund. At that time the College Fund was worth about $35K for a 3yr enlistment and more for a 4yr, 5yr, 6yr commitment.
Out of more than 200 jobs only 3 gave me what I wanted.
If you want to be a MP, they require a 6yr commitment as so many people want to do MP that they do not have a need for new ones. The best part is if you fail out of MP AIT you get shafted into another job and your 6yr commitment stays. Lots of technical jobs which are highly sought after have minimum commitments of 4yrs or more and you can forget about getting a bonus.
OK, so there I was pissing my pants at age 17, green as could be, while being screamed at 16hrs a day, sleep dperived, overworked, stressed the F out. Wow! Was not fun at all.
The maximum age you could join while I was in was 32, they have since increased the age much much higher. The oldest in my basic training I beleive was 28. Unemployed family guys with kids who needed a job, healthcare and food on the table for the family.
Quite a few 22-24 yr olds with mountains of student debt and no job as well. These guys were the brightest of the bunch and held rank over us as college credits can bring you in as high as E-4. That is a huge pay differential over a E-1.
Questions?