We went a few years ago. My then-5 year old (who is borderline nervous on HM) enjoyed it. My 6 year old didn't like it. My 30 year old self haaaaaaated it - it was way too scary for me.
I would say it depends on the personality, not the age.
BTW both my kids have gone on the HM from when they were 3. One of them never was scared, either then up until today. The other was fine with it until she turned 5, and then she had some sort of fantasy jump in her mind and she was terrified of it for a while. She's OK with it now, but still a little wary. They do go through developmental stages, so just because they're OK with it now might not mean that the fear might not resurface later on...
I also loved BTMR and Space Mtn as a kid. I couldn't fathom why my mom didn't like them. They were the BEST! Now, with my adult self I'm a bit nervous to go on them. I feel they're almost too risky - even though I know they're really not, but still. A kid (or adult) could be thrown by the spooky atmosphere, ride mechanics, fear of motion sickness - different stuff. It all depends.

BTW both my kids have gone on the HM from when they were 3. One of them never was scared, either then up until today. The other was fine with it until she turned 5, and then she had some sort of fantasy jump in her mind and she was terrified of it for a while. She's OK with it now, but still a little wary. They do go through developmental stages, so just because they're OK with it now might not mean that the fear might not resurface later on...
I also loved BTMR and Space Mtn as a kid. I couldn't fathom why my mom didn't like them. They were the BEST! Now, with my adult self I'm a bit nervous to go on them. I feel they're almost too risky - even though I know they're really not, but still. A kid (or adult) could be thrown by the spooky atmosphere, ride mechanics, fear of motion sickness - different stuff. It all depends.