The problem is that brett and reduction aren’t that simple. You can’t say that brett equals one thing and reduction equals another.
First of all, reduction just means something is not oxidized. But for argument’s sake, let’s say that when we’re talking about wine we’re only talking about sulfur when we talk about reduction.
There are many different types of sulfur compounds and depending on their concentration, the same compounds can even seem to smell different. So there is a range of aromatics that might be caused by reduction. In general, sulfur compounds stink, but they don’t all stink the same way. I think there are somewhere around a hundred different sulfur compounds that have been found in wine - not entirely certain about that though and don’t feel like looking it up at the moment. In any event, there are many of them and depending on what combination they occur in and their concentrations, they manifest themselves differently. Some react with oxygen when given air and form larger compounds that aren’t as volatile - that’s what “blowing off” usually is. The sulfur is still there, but as part of a heavier molecule that is less volatile and therefore harder for us to detect.
Without adequate oxygen, nitrogen, and nutrition, yeast can instead use available sulfur and can make hydrogen sulfide, among other things. In addition, some strains of yeast are more prone to form sulfides than are others, and sulfides generally stink. Racking, topping up, filtering, and bottling allow the incorporation of oxygen and thus help to prevent sulfide formation. The addition of elemental sulfur, either in the vineyard or to the wine on the other hand, can do just the opposite. Those sulfides don’t blow off with aeration and they aren’t easily oxidized, again, they simply form disulfides or heavier, less volatile, sulfur compounds for which we have higher sensory thresholds. Some however, can precipitate with copper, so copper treatment during the winemaking process is used. Pennies don’t have that much copper these days, but if you have an old one, or some copper pipe, that can work.
Some people don’t think that a little stink is a flaw and in fact, sulfur compounds aren’t always unpleasant - the petrol scent of Riesling is the most obvious, but the yeasty or bread-like aromas from aging on lees that you get in whites like chardonnay or sparkling wine is also a reductive scent, and what I think people sometimes call “mineral” notes in highly acidic whites may be yet another.
Finally, some grapes are more prone to reduction than others. Syrah for example, or Mourvedre. It’s one reason for adding Counoise in the south Rhone, because that grape is prone to oxidation. One of the developments in the south of Spain recently has been the production of Monastrell with careful attention to prevent reduction - it produces wines that are completely different from the stinky wines of yesteryear.
Brett is just as confusing.
it’s all over everything. There’s absolutely no reason to think it’s particularly associated with Chateauneuf du Pape - that’s a huge misunderstanding. It’s associated with all wines around the world, particularly the “peasant” wines. Until recently, CdP was one of those regions. The great influence of Parker on CdP has been cleaning up the winemaking. Same in the North Rhone and in Bordeaux. Same in most of Spain and Italy and certainly in central and eastern Europe.
In the US, UC Davis preached clean winemaking, so brett wasn’t such an issue but that doesn’t mean it can’t be.
With a little oxygen and some nutrients, brett of some strain or another can grow using the alcohol in the wine as a source of carbon, some amino acids in the wine for nitrogen, and even some of the sugars in the wood of the barrels, particularly the toasted woods.
So the wine needs to be able to eat up, or use any available oxygen, so as to deprive the Brett of any ability to grow. Also, Brett grows quickly at temperatures above 60 degrees F, so if you have any in your wine, keeping the wine cold is important. So to the OP’s question about wine age - it can matter quite a bit if brett has been allowed to flourish.
High acidity can inhibit some brett growth. But “modern” wines, with higher pH, higher sugar levels, and in some cases, reduced use of sulfur, all contribute to the growth of brett. So even if the winery is clean, the winemaking style may contribute to brettiness in the wine.
And again, because there are so many strains of brett, and because it manifests itself differently depending on what it is associated with and how concentrated it is, it’s pretty much impossible to say that the absence of a particular aroma is definitive proof of the presence or absence of brett. It can contribute to the band-aid, leathery, smoky, earthy aromas that some wines have, or even some of the buttery notes, or it can be awful.
Because both sulfur compounds and brett can stink, they both can produce what people call a “barnyard” character, but again, that’s imprecise and there is barnyard and there is barnyard.