In reply to purplemonkeyelephant:
Depends what you mean by "dynamic resistance". I know, that 20 year old heavily used ropes almost certainly hold the requisite number factor two falls, so are perfectly safe. You'd maybe not want to take the number of factor two falls in the manual minus one, but they're safe.
What's probably the case is that they stretch less, so the fall is harder on your body. If you're expecting to take really big falls (Rhapsody level), use a new rope. And more importantly, let any rope that has absorbed a huge fall rest for up to two weeks to recover its dynamic stretch.
Based on research done by the alpine clubs, it is also true, that ropes shear more readily when loaded over a sharp edge after relatively little use. This is where your 50 descents might come from. The conclusion from that research was, that for a maximum of safety you'd want to get a new rope every week. Since nobody has that kind of money, another way of looking at it is that loading the rope over an edge is always dangerous and where talking about a subgroup of duller edges where in some cases the new rope might hold while the old wouldn't. It isn't as if one would kill you, the other wouldn't, but that with a brand new rope your chances are better in a specific range of cases. Outside that range you're dead or fine either way.
As the reduction is asymptotic, it doesn't matter all that much whether you replace your rope after 4 or 5 years.
If you go by handling and gut feeling, you'll be replacing your rope far sooner than necessary anyway, so it's best to stick to that any not worry about marginal properties in special circumstances.