Here are four main alternative programs for you to take a more careful look at.
(18-24) age range for Americorps
City Year (17-24) works in U.S., London, and South Africa
Student Conservation Association (SCA) (over 18) last 12 to 52 weeks.
Finally, the Peace Corps - overseas for 27 months. This one has students and older professionals (you mentioned a maximum type age)
Check out those sites and the information they have - hopefully one of those will help to fit the needs you have.