Part of the reason that Michael J. Fox was able to play a teenager in the 'Back to the Future' movies so convincingly is that he's a wickedly talented actor. Another reason is that American movies, for reasons that have never been clear to the rest of the world, always seem to cast people in their late 20s (or even early 30s) as teenagers. The third reason, though, and perhaps the most compelling, is that Fox simply didn't look like a fully-grown man at the time. He was too short!
At the time of making the movies, Fox was only 5 feet and 4 inches tall. It's even possible that he's become shorter since then - Parkinson's Disease can stiffen bones and prevent people from standing at their full height. The illness hasn't stopped Fox from working - in fact he even turned it into a feature of his performance during his memorable cameo appearances as an evil lawyer on 'The Good Wife.'