The term for retention of larval traits by a sexually mature axolotl is neoteny.
Axolotls are salamanders that become reproductively mature while retaining features normally associated with a larval stage. They keep prominent external gills, a finned tail and an aquatic body form instead of undergoing the dramatic transformation into a terrestrial adult seen in many related salamanders.
This is often called full neoteny because the axolotl normally remains in its larval-looking form throughout life. The process is a type of paedomorphosis, a broader evolutionary pattern in which adult organisms retain juvenile characteristics. Neoteny is therefore the more precise answer for the phenomenon described in this question.
Axolotls can sometimes be induced to metamorphose under laboratory conditions, including through hormonal treatment, but that is not their ordinary life history. They are native to the lake-and-channel environments around Mexico City, especially the remnants of Lake Xochimilco. Their striking regenerative abilities have also made them important research animals.