At the very least, a godparent needs to be an adult, able to take on adult responsibilities for the child in question. There is no exact limit in age for a godparent apart from this requirement.
A godparent effectively takes over some responsibilities for the child that the child cannot take on at the moment. Traditionally, the godparent sponsors the child at baptism and makes a profession of faith for the young one. Nowadays, this role has lost its implicitly religious implication and refers to any adult chosen by the parents to take an interest in the child's development and upbringing.