No limit for the front underwear bit, as far as I know. I bought a bra in there when I was about 14.
I think there's a limit for the back area with all the sex toys etc, which is 18 I think. I don't think they'd ask for id if you looked about 16 or over, though. My boyfriend (now ex) bought me something in there when he was 17. When I recently bought a bra in there, I was told of an offer on vibrators at the till, implying she was prepared to sell them to me, and I am now 17.
It depends on the shop in your area, I'd imagine. Some probably have a strictly no under 18s policy on the whole shop. I know online you cannot buy anything (even underwear) if you are under 18 .