Looking at the Rapture numbers, iOS 7 looks like it's just under 5% of our audience, and iOS 6 is about 1.5% of users. I would also assume that the revenue per user is likely to be slightly lower for people with older devices. So sure, you'd be missing out on some revenue, but probably not very much.
I heard Apple still has plenty of early enough adapters that you should be fine. Besides, there's a decent chance that some of the people on ios7 probably wouldn't be interested in games anyways. I myself won't update my IpT5 to past ios7, but then again, I haven't bought games in awhile.
Well, you certinly miss out a large part of audience. Although, I am not sure about their paying capacity. Try out this thread, a guy asks the same question.
Perhaps not qualified to speak on this subject, but glancing around these forums shows that those who deliberately stay on older versions of iOS may disproportionately be gamers or other hardcore users unwilling to relinquish their now out-of-date purchases. So, it's a small audience, but possibly one more likely to buy new apps if any are available. Or it could be that they're like my wife, who has never knowingly updated her iOS and only downloaded about six apps in four years.
