I hunt some every year with irons, and apparently my eyes are still good to go for that purpose even at 62. But I also think a lot of hunters think their eyes aren't good enough for irons anymore mostly because they haven't shot much with irons for a while.
I don't hunt with irons because I think they're better than a scope. Instead I hunt with them because it's fun, in several ways. In fact one of the things I'd forgotten about hunting with irons was relearned about a dozen years ago on an all-irons safari in Africa, where I took a Cape buffalo and five head of plains game. I relearned that hunting with good, rugged iron sights was more pleasurable not just because of the hunting, but because there wasn't any damn worry about cleaning scopes, or worrying if they'd get knocked out of alignment, or suddenly go batschidt. Instead I just went hunting.
As far as the distance where I "need" a scope, it isn't so much distance as overall conditions. On that safari the six animals were taken at ranges from about 75 to 225 yards, and the only miss occurred on an impala where I hurried the shot, because some nearby giraffes had spotted me and were getting nervous, which made the impala nervous. I got nervous, and simply pulled the shot. But all the others went where aimed.
Have killed a few other animals at 200-225 with irons over the years, but my longest was 350 on a bull caribou. Conditions were right (open country, decent light, stationary animal, prone position) and the shot was actually pretty easy.
