That's a very difficult question -- it's really impossible for researchers to know exactly how long certain dinosaurs lived, let along how old they were when they reached adulthood, because we've never had any live dinosaurs to study. But, researchers make educated guesses to try and figure out how old dinosaurs lived. Some researchers think that dinosaurs have a lifespan similar to modern-day mammals or reptiles of the same size, and that smaller dinosaurs probably had shorter lives than larger dinosaurs, but there is a lot of lifespan variation between all of these animals. But going off of these theories, triceratops may have lived to be 30 to 80 or so years old, and if this were the case, they probably reached adulthood (and could mate and lay eggs) when they were 5 to 10 years old (based off of when similar-sized mammals and reptiles reach maturity today).