If you're using extracts (LME/DME) your beer will "break down" fairly quick. After doing some research on the topic, I have found that the vacuum drying PROCESS to make these extracts, makes them prone to break down. The sure tale signs are off flavor (that "home brew" taste), and even more easily spotted.. the large bubbles for a head, instead of that nice frothy one we all look for. With all grain, this does not happen. I had a 2 yr old, all grain brown ale last week.. OUTSTANDING, like it was brewed last month. All grain does not break down. If it does, it's super slow, and fairly undetectable.
Last week, we did a taste test. We we have some of our top notch rye ipa in both kegs and bottles. Poured a bottle into a glass, and drew a pint from the keg. The aroma from the bottle totally blew away the kegged pint. Flavor from the bottle was "softer" or more smooth. This was undeniable. Beer/ale conditions much better in glass. It's a fact.
one other bit of advice, we only use the Coopers clear bottling drops. We got away from adding a bag of corn sugar. The idea that you have the perfect, even amount in each bottle, is always best. They're clear (like a halls cough drop) dissolve nicely and leave no residue.