It is weird how this happens. I remember at 18, mum asking if I wanted a party to celebrate my 18th or wait until my 21st. I was so glad I did my 18th, because the social event we all used to go to packed up before my birthday, so I was still hanging on by my fingernails, even then.
By the time I was 21, I had no one to invite to a party! I was lonely and actually placed an ad in the lonely hearts column in the local paper - something I regret, but that's another story.
But by 21, I felt so alone. It's quite a difficult transition. I started a degree course at that age, having fouled up attempts at a career multiple times, so things probably picked up a little after that, but I've often longed for the social scene I had when I was 16-17 and I've had to accept, it's gone.
I'm sure you'll find quiet new friends who share your interests eventually. The pandemic has made it difficult. Happy birthday to your friend! :-)