When do leopard geckos get fat tails?

my leopard gecko is about one month old and I was wondering when his tail is suppose to get fat or start getting fat.about 3 to 4 months old. They will put all their energy and proteins into growing when they are small. It will really fatten up if fed well at about 6 months old. They will also get fat pockets under their arms in the pit area so if it starts getting what looks like bags their don't worry.leopard geckos store their extra fat in their tails.  as long as his tail isn't really thin or getting thinner don't worry about it, it will thicken up.  if it is really thin try feeding him more or supplementing his diet with the occasional wax worm (no more than twice a week)If your geckos tail isn't fat it's a sign that he's not very healthy.
"When purchasing a leopard gecko, especially from a pet store, check for signs of illness. One of the best indicators of health is the tail. A healthy leopard gecko will have a fat tail. Avoid purchasing a leopard gecko with a thin tail, as this indicates poor health."

So it will "start getting fat" when it starts getting healthy!
Here is a picture of leopard geckos hatching..notice their tails are already pretty fat.
http://www.leopardgecko.com/The more nutrition of mealworms and crickets they have, the more the tail will begin to get fat and be fat at about 3 months or so. It has been what we've experienced, with the two leopard geckos we have! It gives them the food they need!! :)i believe that happens as scar tissue after they have lost their tail and it grows back...http://www.anapsid.org/lepgeckbreed.htmlgive it time it's still a baby.  I got some for my neice and it took about 4 months.  When they are young they put all their time and energy into growing, once their growth levels off the tail will start to get fat as long as you feed him a good diet which i am sure you are.