I was a junior in high school. I was in a shop class that was pretty much all guys. The teacher was really laid back and was talking about losing his hair and told everyone to enjoy theirs while they still had it and added that his started with a receding hairline before it got really bad.. So naturally all the guys started inspecting their hair and hairlines. I remember looking at mine in the mirror in the classroom and noticing that one of my temples was actually slightly receded. Maybe NW 1.5 on that side. Another kid saw my hairline and said "Oh man, you're definitely gonna be bald!"
So that triggered my "awareness" of my hairline. Before that I never thought about it. By the time I was 19 or so I had noticeable recession on both sides but still very thick density. I wore my hair down and it didn't look like there was any hair loss at all, so even though I was a bit freaked out, my appearance still looked good.
By 21 I was NW 2.5 and panicking. That led to me going on and off of finasteride multiple times over the next few years. At 24 I met the girl who is now my wife and I stopped worrying about hair loss so much. However, I did have to quit fin for good because it was messing with me, and in fact, I was still dealing with some side effects when I first met her.
Fast forward to 2020 and I'm turning 34 soon and NW 3. No meds. So thankfully my hair loss has been slow. Unfortunately, it has continued to progress, and now I have to be really creative with my styling and haircuts for it to look decent. I'm waiting until age 35-36 to get a HT to fill in my temples, as I believe that's a pretty good age to determine exactly what type of hair loss I will have and to be a little more assured that I won't be super bald at 45 or something.