I have about 10 cases of beer in my cellar right now. I know all about drinking it old. The question of "what's bad?" is a personal one. It comes down to personal preferences, and yes, each beer will age differently. Personally, I don't mind a slightly stale beer, but others do. At a certain point, most beers will go bad. In those cases, I do indeed end up dumping some of my really bad beers that might have been fine when first bottled, but eventually really don't suit my fancy at all anymore. I'd say this happens with about 20% of my old beers. Most of them might be a little "off" but still drinkable. Others might not agree.
If by your palate you must insist on every beer being superb quality, then you probably want to have it all consumed within about the first 6 months from packaging. This will work for most styles. Very high alcohol beers above about 7.5% ABV will last for years instead of months, and something around 7% ABV will be someplace in between months and years. Let your own palate be your guide.
It is in part for these reasons above that I now brew only 2 gallons at a time instead of the 5-6 gallon standard recipe size. Brewing less volume helps me drink it fresher. Even so, like I said, I still do in fact have about 10 cases to spare, and I don't drink it nearly fast enough for as much as I brew. I wish I did but the truth is that I am a little more fascinated by the brewing process and the variety than I care about freshness. I just can't stop myself from brewing more and more. Perhaps I should cut down to just 1.25 gallons at a time. The only problem with that in my view is how little there is of it, so if it turns out really good, it will be gone in the blink of an eye. So we each have to strike our own balance with how much to brew at one time. Looks like your batch size is too big. Brew less volumes. Scale down your recipe sizes. I should do the same thing, but I choose not to. 2 gallons is the least amount I am willing to brew at one time. Brewing is quite a lot of work, and if it turns out really good, I'm not sure when I'll get around to brewing it again.
All good food for thought. Cheers.