Axolotls can reach sexual maturity from anywhere between 5 months and a few years, it all depends on frequency, quality of food, and the water temperatures/conditions. Generally between a year and a year and a half is a good time to judge the sex of your Axolotl.
The first sign your Axolotl is reaching sexual maturity is the coloration of the fingertips. The lighter colored Axolotls will develop a darkening on the fingertips. The darker colored Axolotls will develop lightening on the fingertips. This is one of the fist signs of sexual maturity.
Females, when they are around the age of full sexual maturity, will become noticeably larger (fatter) than male Axolotls. This is due to them being graved. Thier cloacal (private area behind the back legs) will stay relatively small.
Male Axolotls will be slimmer than female Axolotls. Their cloacal will be much larger than those of a female.