Generally, age doesn't really matter too much for feeding- it's all about the physical size of the animal. For example, any axolotl under 2- 2.5cm should either be eating BBS or daphnia. After that age an axolotl can begin to eat live blackworms. Mine took frozen bloodworms at 2.5 cm but I had to chop in up into bite-sized chunks and drop it literally on their heads for them to snap at it. Anything greater that 4cm can easily take frozen bloodworms, but you may need to manually feed until the animal gets used to eating dead food. 6-8cm can eat small chopped earthworms. I would feed chopped earthworms until you are sure they won't choke/have trouble managing a whole earthworm.
As for how to tell the actual age, that varies greatly based on diet and the individual. I've seen people post pictures of their year old axolotls that are the same size as their 5 month old ones. How big are they now? We may be able to give an estimate based off of that.
