How do you tell how old a cactus is?

It's impossible to come up with a precise age for a giant saguaro cactus [Carnegiea gigantea]. In fact, guesstimating ages just can't be done regarding any type of cactus. So much depends upon the individual cactus and upon environmental conditions. But a saguaro cactus that has at least one branching arm and that flowers, fruits, and seeds is at least 40 years old. Not one of the preceding activities takes place for a saguaro before that age. Additionally, it tends to reach its mature height of 40 feet/12 meters in around 60 years.