From visual assessment alone, kerry is right. Axies cannot be accurately sexed until about 1 and a half years of age.
However, if cost is not an issue, and there is a pressing urgency to sex the larvae, there are other tests available. A biological sample (drop of blood, skin or gill tissue) is obtained to run a genetic test (to determine sex). This test can be requested upon consultation with your vet. This is normally done with some species of birds and reptiles but can also be done on amphibians.