The answer is simple. If, throughout it's life, it's been maintained properly, it's worth keeping. If it's been completely neglected, and the steering, suspension, and drive train are worn out, it's ready to be retired.
If it is running well yes. The problem with old cars tends to be finding parts, when a 20 year old car breaks down, or after an accident, it may be difficult to find a part and you may end with the car in the shop for a while. That is usually the only problem.