Big
(1988)

Josh (Tom Hanks), longing for his old life as a child, finds the fateful Fortune Telling machine and makes his wish to be a kid again, just as Susan (Elizabeth Perkins) comes to meet him and realizes that he was indeed telling the truth.

She drives him back home, they say good-bye to each other, and, while walking towards his house, he transforms back into his normal, 13-year-old self (David Moscow), to her amazement.  He enters the house, calling for his mother, and we hear her screaming in joy, while Susan drives away.

The movie ends with Josh and his best friend Billy (Jared Rushton), walking down the street, talking about trivial stuff.
 

Thanks Dr Mat!