This is an extremely difficult question to answer with any certainty. I would guesstimate that well over 90% of pet reptile deaths are preventable in some way, at least theoretically. Many problems can be hard to detect until it's too late. If kept in optimum conditions, and any problems are dealt with quickly, most reptile species can live seemingly forever. But all it takes is one lower respiratory infection (which can be hard to detect, and can be much more harmful than the more common upper respiratory infection), to go unnoticed, and many animals may never recover. Add to this, even if certain problems are detected, that proper treatment is difficult to determine and administer properly, even with expert herpetological veterinary care, and what exactly qualifies as "natural causes" its completely up in the air. I always think that any reptile death I've seen was preventable, at some point, though allot of times by the time the end is near its too late.
But to answer the original question, they slow down, start eating less, and become less active. Very similar to the actions of most elderly animals. And just like most other pets, their care should evolve as they age. The most common mistake in my opinion, especially with snakes, is overfeeding. Too many people want to watch their snake just eat a large prey item every week. A healthy, full grown, adult snake doesn't need to eat anywhere near that much. And for some species it can be downright detrimental.
But, as I said, there its normally some actual "cause"of death, so, there would be other symptoms associated with that actual cause.
