There's no source I'd believe on this. Kids don't become shy when they turn 13.
Hide in room, grunt when asked questions, moody, "disrespectful to their elders", that kind of thing is all "less social".
Although I can think of one specific context where I would say "shy" for a bit: suddenly developing a new axis of emotional drives (lust) that they have to learn to navigate socially.
As someone who happened to hit that inflection point at the summer after high school I consider my cohort extremely lucky.