Flight of the Navigator (1986)

"David Freeman is eight years late for dinner, and his family wants to know why. But David's got an absolutely fantastic excuse."

12-year-old David is accidentally knocked out in the forest near his home, but when he awakens eight years have passed. His family is overjoyed to have him back, but is just as perplexed as he is that he hasn't aged. When a NASA scientist discovers a UFO nearby, David gets the chance to unravel the mystery and recover the life he lost.