I have two axolotls roughly the same size. I measured one this morning and it's around 12cm in length. Can this help at all when decided how old it is? I forgot to ask at the petshop which I think is a big mistake and I would love to know it's age.
You can never be 100% sure. I have 4 axies that are all the same age and got 4 distinctly different sizes at the moment- one is about 10 cm, one 9cm, one about 7cm and the other 6cm. All were born on the same week, so it depends on how much they eat and how fast they develop