I've got the 22 going on 12 syndrome when it comes to women. Lack of experience certainly seems a likely candidate for the 22 going on 12 syndrome, but I don't think it's the whole story. At the root, it seems to me, is a fear of rejection. Loud and clear. I've experienced that phenomenon most of my life.
Rejection tends to decrease self-confidence to an exponential degree.
Especially if the rejection is for non-specific reasons, or a broad category. It compartmentalizes yourself and other people both, but it's pretty common, especially if you have low self worth to begin with for other reasons. But there are people who really are shy, but not in the conventional sense. People who will pretend and put on masks and what not to the ends of the earth, anything to avoid showing the real person inside. To me, that's very shy.