I think they are adults in typical chibi Japanese design, but I think they made characters cute to appeal to children as well. The characters are living out on their own so there's that. However, I think the ambiguity is meant to create a sandbox environment so Animal Crossing is a game anyone can play, regardless of age. The beauty of Animal Crossing is that you can decide whether or not to roleplay whatever you want. If your OC is an adult, then it is. If your OC is a child, then it is. It is entirely up to the player.
