A lot of people seem to think it matters, but in reality it doesn't. The 50 1.4 has been the same lens for a long time. The age really doesn't matter, the amount of use or abuse totally does. The sharpest lens in my bag for long time was the venerable "Magic Drainpipe" 80-200 f/2.8L. It was 15 years old when I bought it, and 21 years old when I sold it to someone who now regards it as their sharpest lens. I purchased the lens used in excellent condition, and sold it to a friend in the same condition. It had always been stored in a bag, the caps used, never abused or bashed around in any way, it was in nearly the same condition it left the factory in some 22 years ago. So you could purchase a lens like that with a 22 year old date code, or you could rely on a date code and purchase a 5 year old lens that had been used by a photojournalist that never used the caps, stored it on the body, and let the body/lens rattle around in his trunk.
Age doesn't matter, use does. Just realize there are some lenses that have been updated and they usually have a number such as a II after the name. The newer 70-200 f/2.8L II is a better lens than the 70-200 f/2.8L, by a little, but the older 70-200 f/2.8L was the de facto standard for that zoom lens in it's day, and it's still a pretty good performer today. So what I'm trying to say is: Decide what focal length(s) you are looking for, decide what your budget is, buy the lens that fits your budget, whether that's a used lens that's 20 years old, or a brand new $$$ lens.
The Canon Camera Museum
has all of the information you need to tell ages and models on lenses. After that do some research on each lens as far as sharpness, etc is concerned and go from there. I wouldn't worry about age one iota, as long as the lens was considered a good lens back in the day, then it's still a good lens today, as long has it's been cared for.
Rule books are paper they will not cushion a sudden meeting of stone and metal -ekg-