I'm not actually sure that short hair will make a guy look younger or not. However, if you had long hair and everyone was so used to seeing you with your hair when you cut your hair, it will change your appearance. This can give the illusion that you look younger.
Some people just look younger, to begin with, and no matter if they keep their hair long or short they will still look young. While other people don't look as good in short hair so they keep their hair a bit longer. Having short or long hair will depend on your face. If you have a full round face it is better to keep your hair a bit longer. This will hide some of the roundness in your face. But if you have a slimmer face, then the short hair will bring out your facial features.
I do know if a man had a mustache and decided to cut it off, this will make him look a lot younger. I'm not sure why this one happens but when it is gone, it seems like they have a babyface now. So if you cut your hair short and shaved off a mustache, then your answer is yes, this will make you look younger.