The short answer is that no-one really knows. If an extremely well-preserved skeleton of a triceratops is ever found then palaeontologists may be able to come up with a more accurate number by analysing the growth pattern of the bones, although this will tell us far more about how long they took to reach maturity rather than exactly how long the lived for. Until that happens we are left with only speculation as to Triceratops' lifespan. So the basic answer is that no-one knows. But it can be fun to try and guess.