Question: how old are big cactus?
Answer: By 95-100 years in age, a saguaro cactus can reach a height of 15-16 feet, and could start to produce its first arm. By 200 years old, the saguaro cactus has reached its full height, reaching upwards of 45 feet tall. Some saguaros have been seen with dozens of arms, while other cactus never produce a single one.