In most cases, people who travel abroad to do volunteer work are high school graduates or students. But that shouldn't scare you off. Even in the adult age, you can do this step and volunteer abroad.
#1: One is never to old to get socially involved.
No matter if it is children in an orphanage in Bali, turtles that need to be rescued in Sri Lanka, or local in the Philippines who need help with renovation work: everyone is looking forward to each single volunteer! Age does not matter at all. People or animals who are hoping for help have no ideal how their helper should look like. They don't care if you are male, female, tall, small, old or young.