Well, I don't have scientific fact to back me up and I have never personally witnessed the actual breeding act...by a deer.
But after sitting through dozens of November bow hunts through the years, I will offer these opinions...
1. In years past I hunted areas that rarely had bucks that lived to maturity, whether you consider maturity 3-1/2, 4-1/2 or older. Virtually any buck seen was at best a 2-1/2 and then you better not say anything because all the neighbors would gang up on him and he would be hanging in someone's front yard a few days later, no joke. Additionally, the buck to doe ratio was such that there is no way in my mind that one or two mature bucks could "catch up" to all of the does in the "magic window", even if they were there.
So whatever breeding went on had to be predominantly from 1-1/2's or or the occasional 2-1/2's or there weren't going to be any fawns in the spring. (FYI, I am intentionally discounting the potential for aliens to have landed on the earth and had their way with the female deer in the area.
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2. Having now hunted an area for about 10 years that has a much healthier age structure, in that there are multiple bucks "around" that would be at least 3-1/2 years old, I will say that oftentimes when a detectably "hot" doe is spotted there is a mature buck in her wake. But not always, I have seen little guys hot on the trail too, with no big guy in sight. Now whether they ended up "cashing in" or not I cannot say, but from where I sat, things were looking, umm, well... "up" for junior when I last saw them.
So my sense is that some younger bucks are breaking through every year, even where then are older bucks present.
I think that if an older buck catches up to a doe at "that time" he is very likely to have "dibs" v. the little guys, but I still believe that some young ones breed every year too.