Wicca is about a lot of things, but I guess I would say it's mostly about seeing the divine in nature, feeling how the life force flows through all things, and becoming personally familiar with the cycles of nature - the seasons, the phases of the moon, the cycles of birth and death. It's about magic, which is just learning to feel the currents of energy that connect us to everything around us, and to other people. And it's about seeing the divine as both Goddess and God, working together in balance and harmony, just as women and men should be equal partners in society. There's a lot of overlap, and not everyone really agrees what the differences are. Basically, Wicca is a particular kind of pagan witchcraft.
Paganism includes a bunch of different kinds of nature-based religion or spirituality.
Witchcraft has something to do with folk magic, and again, there are a bunch of different kinds of witchcraft, and not all of them are pagan. Wicca is a specific magical nature-based religion. There are different kinds of Wicca, in the same sense that there are different denominations of Christianity, for instance. Traditional covens usually will only train adults, and in fact it is illegal in most states to provide religious training to anyone under 18 without their parents' permission. There are some things about Wicca that may make more sense at age 21 than at age 14, but there's no harm in following the parts of the religion that make sense to you, whatever your age.