Well here’s the technical bit, wines only turn to vinegar when they are fermented by acetic acid bacteria known as mycoderma aceti. While loads of wines have a bit of acetic acid in them, it tends to be unnoticeable and doesn’t affect the taste of the wine.
But what happens to wines that age poorly is that they cook, yes cook, now this can happen for many reason but mostly it is because wine is stored in the wrong places a cupboard somewhere or a fancy rack in your kitchen, or the worst culprit of all above the fridge. Sound familiar?
But that doesn’t explain why some wines get better with age. In general, that’s due to the wine’s structure: acid, tannins, and sugar, as well as the quality of fruit in the wine. It’s a result of all these factors and more such as the grape variety (some age well, others not so much), the terroir (some regions make more structured, and therefore more ageworthy, wines) and, of course, the winemaker. Some winemakers aim for an easy-to-drink, lush style of wine that may not improve in the bottle, generally this is the most common sort of wine you will come across, while others may want a brutally tannic young wine that can last, and evolve for ages. As a result most wines available these days are designed to be drank young or over a short-term of 3-5 years, before they start to become vinegar!!