iPhones will not connect 802.11n

I am new to the airport family so my appologies if my problem has been solved (I didn't find anything searching) or if its obvious and simple.

I currently have an airport extreme broadcasting a 802.11a/n - 802.11b/g wpa2 network. I also have a wired express creating the same network to extend it (thanks for the help with that, Bob).

First, i noticed that my MBPs were connecting to the N network but only 2.4ghz, so I set it to broadcast a separate 5GHz network name. after that, the MBPs were connecting to the 5ghz network without a problem.

So, when I check the clients on my network, I notice the iphones ( 4 and 4s) in the house are only connecting on 802.11b/g not a/n. This happens even if the iphones are only a few feet from the router.

Why are the iphones not connecting on the n network? Is this a setting issue or simply part of the technology I am misunderstanding?

Thanks.