Unless you've had your gecko since it hatched, it's going to be difficult to pinpoint its age. The best you can do is estimate the age based on your pet's length from nose tip to tail tip and its weight in grams. Bear in mind that nutrition and the correct housing environment also influence the gecko's growth and overall health, so don't put all of the emphasis on size if you're trying to decide when to schedule its birthday party.
Remove your gecko from its tank and gently place it on a flat surface alongside a ruler or even on a ruler. Avoid picking it up by the tail, because the tail is fragile and easily broken off. If yours is a leopard gecko, which is the most popular lizard among U.S. herpetoculturists, and if it is an adult male, it may measure from 7 inches to 8 inches long from nose to tail. The adult female leopard may measure from 8 inches to 10 inches long. But remember, those measurements are approximate for full-size geckos and don't really tell you if you have a 6- or 8-year-old or a 1-year-old.
Weigh your gecko, using a gram scale. An adult male that has reached sexual maturity – roughly 8 to 9 months of age – should weigh at least 50 grams. An 8- or 9-month-old female should weigh at least 45 grams. Again, weights are approximate – they could be higher or they could be lower -- and are in no way positive indicators of your pet's age.
Check out your gecko's colors or patterns. As a rule of thumb, younger geckos are more colorful than their more mature brethren; the babies are banded or have big blotches. As the gecko matures, the distinctive leopard-spot pattern emerges.