We're also expecting our first child in the next month. I plan on taking her when she's 12 +/- provided she's a football fan.
I'm 31 now and started going to games with my father when I was 9 (just in time for the Glory Days). We sat in the cheap seats then and I loved it, but it's a different place now. I also went to Foxboro when I was 12 and was terrified for 3 hours after watching bottles get thrown at the Bills fans' bus as they pulled in the parking lot.
That said, I'd wouldn't take a son under 16 to sit in the endzones or upper decks now unless we were with a large group that took up multiple rows, and a daughter... never. In the club seats I'd feel more much more comfortable at 12.
On the flip side, my wife and I are both big baseball fans, and I have no problem taking an infant to MLB game.