As an ex-Volunteer, I feel part of the privileged people on Earth, it is such an enriching experience!
Anyone can become a Volunteer, really. Only interest in other countries & cultures, an open mind, the will to learn (anything!), the will to help, are needed. No specific skills are necessary, you would always find a project that suits you whatever age you are.
Some people enroll for the adventure when they are young and use their gap year to do it for example, others would have a career break or would feel the need even once they are retired.
It is a wonderful human experience where you wake up every morning knowing you will learn something about the World we live in: a new word, a new culinary specialty, a new custom, something about yourself, or even, meet new people…