Do you monitor what your 13yo reads?

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DS is an avid reader but of late has been re-reading books that are a bit easy for him IMO opinion. I'd like him to challenge himself a bit so I've been looking at the Book People "Young Adult" section which they say is for 13+. Thought I'd treat him to some reading material for the holidays.

A lot seems to have very adult or dark themes. I'm torn between thinking he's not old enough and I should protect him from this stuff (wouldn't let him watch an 18 film) and maybe it's time he was exposed to a more adult world.

What does everyone else do?

No, I don't.
I seem to remember when my eldest was that sort of age, I'd ask him what he was reading every now and then and what it was about, etc., but more as part of the whole family conversation as I was still involved more in younger ones' books, but by time dcs 2 & 3 got to about 10, they chose what they wanted from the library.
Youngest one,(now 12) does quite often ask older sister about something she's read - often when we are in the car, so if there's anything disturbing her, I guess it would come out then.