It depends on the person for me.
When I was younger, I used to have a crew cut every Summer (my mother forced me to). This happened from around when I was 5 until around the age of 11.
It didn't look very cute anymore once I gained weight. (I started gaining around the age of 10.) So, picture a fat, bald kid with a space between his two front teeth who wore nothing but T-shirts, shorts, and flip-flops.
...Looking at old family movie clips, I bury myself into my arms, wondering why.
Once I convinced my mother to give control of my scalp to me, I just had it at a short length for awhile, cutting it once it grew a bit longer than normal. They drew the line at long hair, though, I didn't much care.
Skip to about age 14 or 15. You guys knew of my existence at this age. It was around this time where I argued with my parents for long hair.
Mom: "Boys aren't supposed to have long hair!"
Dad: "...You won't look very professional."
I constantly argued it, pointing out the fact that my father had long hair when he was my age. So, I just stopped getting my hair cut for awhile. They finally got the idea that I wanted to do what I wanted to make me happy, not because I wanted to be a part of what's in or not.
My father says it's fine for me, now, but my mother still hates it, and personifies my hair whenever I walk across my room.
"Ugh, it's like a shaggy animal on your head. Cut it."
It's at the point where if it's wet, it passes my shoulders. I want it long enough to reach my elbows, then I'll get it trimmed a bit.
So, IMO: Fat guys with very short or no hair = ....ehhhh, you need to have the right face for it.
Thin guys with short or no hair = fine
Girls with long hair = good
Girls with short hair = good
Girls with hair long enough to sit on = crazy.