What film is this?

what film is this its probs a 80s film and its where a lad builds a growing up machine and his friends tries it and grows older into his 20s 30s then the machine breaks and he has to be an adult and he falls in love with a women. his friend tries fixing the machine using water melons he finally fixes it and the lad goes back to a boy at the end of the film the woman he fell in love with goes to his friend and becomes young again and at the end she walks into the cl room about 14-15. if anyone can think of what the films called please tell me! thanks!