18 Again! (1988)

Jack Watson is an eighty-one-year-old businessman, who is not exactly liked by his son, and who is pushing his meek grandson, David to excel in athletics as he did. At his birthday party, before blowing out the candles he's asked to make a wish and his wish is to be eighteen again. While out with David, they get into a car accident and when he wakes up, he discovers that he is now in David's body, and David is in his body, which is comatose. He then goes out to enjoy himself, but at the same discovers just how tough David has it in school and finds out how his girlfriend, Madeline feels about him, and also how his son feels about him.