IPhones will not connect 802.11n

Question

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.

Answer 1

sorry. I didn't mean to disable SSID broadcast; my experience with doing that is it has never mixed well with my macs. SSID broadcast is enabled.
I was referring to when you enabled the setting on the wireless tab in the "wireless network options" dialog box that will enable broadcasting a second SSID for a 5ghz only network (along with your regular SSID broadcast). For example, the two broadcast SSIDs would be;
     phil178821 (which is set to 5ghz n and 2.4ghz b/g/n)
     and
     phil178821 5Ghz (which is 5ghz N only)
What I was trying to described before is if I am sitting close to my router (15ft) and I am broadcasting just one SSID (my normal 802.11n - 802.11b/g/n), my MBP will connect to the 5ghz 802.11n. If I moved up three floors in the house, my MBP will change connection to 802.11b/g/n. With that, my understanding is that it is now connected via 802.11n 2.4ghz. Because it defaulted to the 2.4ghz frequency, I figured I was out of range of the 5ghz network. However, if I go into my AE settings and enable broadcast of a second SSID for a 5ghz N network (as described above), my MBP will pick it up at full strength and connect to it.
So I guess my question is. Why will the MBP opt for 2.4ghz when I broadcast  one SSID, if I know 5ghz is still in range since the MBP will connect to it if I enable it with a second SSID in the AE options (recall, the MBP will chose the 5Ghz frequency closer to the AE, e ven when only one SSID is broadcasted)?
hopefully I am explaining this well enough. Thanks for your help.