Axolotl are born with fluffy gills. The length of the critter is used to determine an approximate age. Even this isn't super accurate since length depends on how well they were fed and even natural differences from one axolotl to the next.
Edit:
Just looked at your pictures. The gills are abnormally small and he is very thin. I would check the water parameters (ammonia, nitrite, and nitrate) as well as temperature right away. Also, if you haven't already, give this web page a look
Axolotls - Requirements & Water Conditions in Captivity. It will give you lots of useful info on how to take care of aquatic salamanders.