I think there are several causes. I actually think hereditary is the least of the causes (but then I'm a "Biology is not destiny" person - don't worry, though, I do NOT think sexual orientation is a choice...)
In my case, it's pretty much pinpointed. I was not shy as a preschooler or even in my first few years of elementary school. It was just after being picked on by nearly all the other kids and even some of the teachers for every little thing I said or did, even down to the way I talked and the way I walked, that MADE me shy. I had a problem with people being hostile to me because of my views on things (I still have a problem with this - it's why I'm extremely hesitant to talk about politics - most people I've known have had views that were polar opposite of mine - and were very, er..., forceful in letting everyone know that fact!)
Also, being seen as not good at anything caused me a lot of anxiety that has had a disastrous effect to me in my career. I did have one steady job that lasted quite a while but when I was laid off from it, it was back to the old "can't get or keep a job" routine. I do get temp jobs but haven't had a steady job in 4 years.
And, oddly enough for a shy person, I have no problem with crowds, as long as we're not packed like sardines (in which case it's more claustrophobia than shyness that kicks in - I'm 5'3" and can't breathe when packed like sardines around people who are mostly taller than me.) If I don't have to personally interact with them, I don't mind crowds. I love concerts and music and ethnic festivals, and those are often crowded. (Haven't been to any in a while though - not many of them are free!) But yeah, my problem is when I have to actually interact with people - afraid I'll say the wrong thing and they'll be hostile toward me or something like that. And in my case, age has NOT affected my caring what other people think. I'm 46 and I care just as much about what other people think of me as I did when I was 16.