Each phase in my life added new relationships which added new anxieties. My only coping method was avoiding. So, eventually I was avoiding more and more of my life.
I don't think it's age that causes less or more anxiety. I think once you know you have social anxiety, you can start to seek help. I think knowing you have this condition is the starting point, no matter what age you are.
For us older people (I don't like to consider myself old, but probably on this site I'm ancient :-) ), we didn't have all the information when we were high school or college age, so we coped with it.
I don't even think the "school counselors" knew what it was. Nowadays, most kids know what ADHD, ADD, OCD, SAD, ect are (it seems very common, maybe too common). I remember when I was in high school (maybe junior year) I was almost having a nervous breakdown. I didn't have anyone (I thought) to really talk to. My mom was overbearing, very critical (she wanted the best for me, but it's hard to be perfect as a kid). I went to the school counselor in tears. I didn't know what I was feeling, didn't know why I needed to talk, but I did. It was hard for me to even go talk to him. Anyway, I told him specifically, do not tell my mom. I look back now, and realize my mom had anxiety, too. She didn't want anyone knowing about our business. Well, that night my mom gets a phone call from the counselor. The rest is history. I could not trust any one in authority. I should have gotten help then. But didn't, and I went through the rest of my life...coping.
I found ways to minimize my anxiety, which unfortunately didn't resolve my anxiety, just masked it, kept me from facing my fears. And as I got older, my thought patterns got stuck in a way that was harder to change.
If you are younger, and know you have social anxiety, you have a head start to change your thought patterns.