Mine hit the fan slowly. Slight recession, got increasingly thinner after late 30s or so. Like Glenn Danzig. It was so gradual I didn't really notice anything other than my ponytail getting skinnier. I got really depressed over it last year, and I'm sure it fueled part of my breakdown/revelation. I just looked at a pic of myself a year ago Halloween, and couldn't believe how much came back due to HRT. It has mostly all come back after 9 months, and I see thinner hair on a lot of ciswomen now. My problem now is that I always kept it long, so I have a lot of hairs on top that have umm, a lot of catching up to do. Feeling my ponytail getting fatter is nice. I'm not sure I'll ever catch up to my wife, but she has extra thick hair.
My father was pretty far along in his early 30s, and my mom's brother had thinned out much like I did by the time he was 40. He never went completely bald, but his is quite thin. I'm sure I was genetically predisposed to follow that pattern. I hear you're 50% likely to be like your mother's brother in that department, but I think most guys are basically 50/50 to lose hair by 40 anyway. Yet another YMMV thing...