There are a handful of places, Buffalo Co being one of them, that have what I believe is a combo of genetics, not too harsh of winters, food and dirt that make them anomalies. I'd have to think harder on this to completely trust what I'm about to write and I just don't have the time right now. So, take this with a grain of salt, but I don't think 50% over 150 even in the Buff COs and SE Iowas at 5.5 is the norm. I know it never has been on any of the BIG grounds I've managed over the years in C MN, SE MN, N MO or IL. Even on the old Kisky farm in Iowa (one of my clients owns it, has Steve Snow manage it, as he did for the previous owner, but the owner shares all sorts of notes with me on the place), there are some unflipping believable bucks on the place. Heck, the owner sent me a ton of pics of what sure the heck looks like a 2.5 to me that flirts with 200", but I'd say they are under 50% for 5.5s breaking 150, and they are specifically managing for top enders to reach 5.5 (they don't have enough hunters to do it right, but it still is better in those regards than nature provides).
I do agree with nofo that a heck of a lot of mature deer are mistaken for being younger because of "meh" headgear. I'd say every bit as many if not more as young deer with impressive racks being mistaken as older deer.
I think there are already a whole bunch of solid reason listed for why Gunther may be experiencing what he is. Here's two more potential reasons:
Once a buck hits 5.5 in most areas, he's the man. He's in absolute peak physical condition (assuming a lack of serious injuries), he knows what he wants and how to get it. He also typically has very few other 5.5s to compete with. that means he can lay claim to the very Best of the Best in his home range. 5.5 is also the age where a relatively high % of bucks really shrink both their core areas and the portions of their home range they spend much significant time in, though a % will be roamers, they are more the exception than the rule. Considering the average home range for mature bucks is somewhere over a square mile and far from symmetrical, few land owners have his entire home range exclusively on their ground. At 3.5, bucks wander all over their home ranges, as they have to work to get breeding opportunities and try to find areas they can dominate. Your 5.5 doesn't HAVE to. So, the odds of seeing the 3.5s that overlap your property are MUCH higher than with the 5.5s. I'd bet that what those 5.5s are seeing as the "best of the best" is unfortunately on a neighbor's ground. In fact, they may even have less desirable habitat, but way less 3.5 &/or 4.5s to have to deal with.
I know deer don't think like us, but think of it this way. You're a 30 yr old in the prime of your life and you're looking to find a girlfriend. your choices are to go to a college bar and put up with all these college jocks budding in on every conversation you are trying to have with each girl you find attractive or you can go to a bar where there are still a good number of girls, but way fewer college guys making idiots of themselves and horning in on your action. The college guys don't intimidate you in the least, but these jocks are obnoxious, pushy jerks that try to get between you and every pretty girl you see. Where are you going to spend your time trying to find a girlfriend? I'm not saying this is what's happening to you, but that I believe this does happen more than most consider.
The other factor not covered is that mortality rates sky rocket from 3.5 to 4.5 and again from 4.5 to 5.5, particularly when there are good numbers of similar aged bucks.. Buck fights and the rigors of dogging does all fall, the 25-30% weight loss that accompanies it, is WAY harder on older bucks than most realize. toss in real winters and it's even worse. Don't quote me on the numbers, as I read this study YEARS ago, but they did a long study in TX on what % of bucks can make it to 5.5. It was a ridiculously huge high fence area with 0 hunting pressure or vehicle fatalities. I can't remember the exact % anymore, but it was less than 1/3rd. That's strictly from natural mortality and they don't even have winters.
I don't pretend to have enough info to even give a solid opinion on what exactly is happening in your situation, Gunther. I do think the odds are it's a combo of things listed in this thread though (mistaking some 5.5s as younger because of "meh" headgear, having too many younger bucks, some shifting their core areas off your ground as they age, a higher natural mortality than you realize and older bucks just being harder to get pics of)
