"Sorry, but I don't know these friends. And even if I DID know, I'd still say no because, the mall doesn't allow kids under age X to be there without an adult. That's because the mall isn't a hangout, it's a place to do business."
Son: "But these guys go there all the time and they're 11 and nobody says anything."
You: "That doesn't mean that it's allowed. The mall policy is age X to be there without an adult." (Show it to him on the mall management's web site if that would help; sometimes kids need to see these things for themselves rather than hear it from mom.)
Then, to me, the crucial next step is to offer an alternative that does work for you as the parent here: Tell him he can invite his friends to your house and they can top their own pizzas and watch a movie or whatever. That shows that you are willing to get to know his friends and want him to socialize, but on terms that work for you and for his age. Don't hover while the kids are there, and give them space to hang around without you, but do start getting to know them and their parents.
Or offer to take him and one friend to the mall sometime, with very fixed limits on where they can go in the mall without you right there next to them (though you'd be close by). But I would not offer to chaperone a whole group of kids there.
I find the whole "hanging out at the mall" thing to be way overrated.My teen daughter doesn't even think it's interesting! It's a recipe for trouble and as Itscrazy posted below, many malls are actually getting a bit tougher with age policies and, around us, with enforcing those policies at least a bit. Kids (and many adults who dump kids at the mall like it's a tween and teen playground) tend to forget that malls are private property, not public spaces, and the mall can set its own policies, and can ask kids to leave.
Don't let him plead or cajole you into saying yes ANY time your gut says no. You made the right call here. Even if the mall said kids of 11 were fine to wander unattended, you can still say no. Even if you know the friends, you can still say no. Offer some alternatives and remind him that you are actually saying yes to seeing his friends, just not at a mall.
If you think you need an argument to tell him no, remember that you are one in charge here and while it's very good to show a child real reasons for an answer -- in the end, he does not have to approve what you decide. Sometimes the no is just a no. But in this one case you have two excellent reasons--not knowing the kids AND the age limit.