I’ve bought a number of >25 year old frags. The key is to buy sealed bottles and they are usually good. Open bottles are usually bad. Like all rules of thumb, there are exceptions both ways. Generally for a given frag, the vintage is stronger and often a lot more animalic. Thinking of Hamman Bouquet that in its current form is pretty feeble but used to have a very distinctive character. Givenchy Gentleman also used to be loaded with civet but is currently rather dull.