First off, the old clutch issue: If you're having trouble changing gears unless the pedal is pressed all the way to the floor, the diaphragm spring which is what actually disengages the clutch is probably old and weak. As springs do, the diaphragm will eventually wear out over a very long time(stock clutches usually last over 100k miles). If the clutch happens to have been used lightly, its possible the diaphragm will actually wear out before the clutch disc. You might want to consider getting the clutch system checked because it's not a sure thing, but it is likely.
As for wear, what wears a clutch out is when it drags against the flywheel. When the pedal is to the floor and the clutch is disengaged, there is no wear because it isn't touching anything. When your foot is off the pedal and the clutch is fully engaged there is no wear because it is locked onto the flywheel. But in between, as you're pulling away from a stop and slipping the clutch, there is wear because of the friction between the clutch and flywheel. If you rev to high rpm while you're slipping the clutch, the friction is very high and will cause the clutch to wear out much sooner than expected.
I've seen clutches wear out from people revving too high while pulling away. Most people rev anywhere from 1000 to 2000 rpm, maybe even 3000, and the clutches will take that abuse for quite a while. But if you're slipping it at high rpm it will wear out quickly.
Also, slipping it exceptionally frequently can cause premature wear. For example, you're stuck in a traffic jam that is too slow to leave in gear, but never quite stops either so you've constantly stopping and going, slipping the clutch each time to get going again. I've been in this situation in other cars. It results in a high wear, and you can even smell the clutch burning hot.
Good clutches, like factory stock parts or high-end aftermarket pieces, can take the abuse from a reasonable driver for quite a long time, so much so that I wouldn't worry about its longevity. But unreasonable abuse, like revving very high while slipping or slipping it repeatedly will shorten its life considerably.
Here's a good way to check if your clutch disc is still strong. If you want to check for clutch slip, you need to do so in high gears because the clutch is more likely to slip in those gears. Start with your highest gear, 5th or 6th, and floor the throttle. If it slips at all, this will make it slip. Then you can go to the next highest gear and see if it slips. It's unlikely it will slip in 3rd or below unless it's in critical shape.
The reason it slips in higher gears is because the weight of the car is more difficult to move in those gears due to the numerically lower gear ratios. The force the clutch has to hold goes up drastically in higher gears, and is positively tremendous in 6th gear. In first gear the force is must hold is realistically still very large, but nothing compared to 6th gear. Anyway, obviously if the clutch ever slips even a tiny bit then it needs replaced.
