For ones they can actually ride in, about 50ish years! Prior to that, they were smaller and used either as decoration or to add an extra element to festivals and burial rites. In the festival/party case: lifting spirits if it's been particularly bad, or sometimes in the spirit of keeping things in balance, they'd make a game of keeping your balloon level.

For burial rites, it was to symbolism the spirits of loved ones passing on and not remaining trapped on the ground.