Hi Weenie, welcome.
Yes, 11 is right on target. My son started at 9-1/2 !
He's into "that," too, but he does not do it at school and at home, shuts his door.
My husband bought him a book that is quite useful (it's very hard to find an appropriate book for that age--you either get picture books on potty training, or adult way-too-detailed info).
The What's Happening to my Body Book for Boys, by Lynda Madaras
http://www.amazon.com/s/ref=nb_ss_b?url=search-alias%3Dstripbooks&field-keywords=The+What%27s+Happening+to+my+Body+Book+for+Boys%2C&x=17&y=20
We pretty much just gave difficult child the book and let him look at it on his own. husband said he had a few questions, but mostly wanted to read it alone because he was embarrassed. I offered to talk and if looks could kill, I'd be dead by now, LOL!
We read difficult child the riot act about not looking at porn online because A) it's the best way to get a computer virus; B) It's degrading to people in the picture; C) It's feeding into a drug fueled industry and most of the women in the photos don't have a clue what they're doing and they, along with-the men, will end up with-a disease.
We encourage him to look at nude art in museums, and professional, attractive models. We have not encouraged him to buy any guys' magazines yet.
Maybe a cousin somewhere will do that when he's 17, but not when he's 11 !
I would try not to make too big a deal out of it because if you make it a really BAD thing, he'll want to get more involved and do things behind your back.
Best of luck.