Adult men shouldn't have crew cuts. They make you look like a child. When I see an adult man with a crew cut, I always wonder, "Did you get a lollypop with that haircut?".
Flat tops are even worse.
Keep it medium length. Brushed straight back, or parted on the side. Keep it clean and neat. Be mindful of how much product you put in it.
Exceptions to the rule:
Bruce Willis (if you are balding, then yes, the crew cut can look better).
African American men.