Who can be a volunteer?
Anyone with a good sense of humour, an open mind, lots of patience, an ability to speak the language (usually English is enough, but in some countries Spanish of French are desirable) and flexibility. These are the qualities which make a good volunteer. Depending on the programme there may be age limits or other skills required.
The same rules apply if you decide to join organized Travel with Mission trips, but as the group is led by a guide, so there is no need in speaking foreign languages (although it could be useful to communicate with local people).