How old is this kitten? She's not growing!

I've been fostering this kitten for 6 weeks now and she has gone from 1 lb 2 oz  to 1 lb 8 oz. (6 oz gain).  When I got her she was full of energy, fully weaned, and using the litter.  The humane society didn't do an age estimate upon intake.  She just hasn't grown at all. We foster until they are 2 lbs (neutering weight), and I can get them graduated in 3 weeks.  We have decided that we will adopt her since she has become part of the family at this point (although the paper work can't be done until she hits 2lbs so it'll be a few months at this rate!!).  My best guess is that she was about 5-6 weeks at intake.  Any thoughts?

Sometimes you can tell the age by their teeth. Here is an article about it. 

http://www.thecatsite.com/a/how-old-is-my-kitten

I have a foster right now the vet estimated her age at five weeks last Friday but she has a full set of teeth and she is able to jump straight to the couch without help. No five week old has been able to do that. 

You said she ate and played fine when you first got her. How is she eating now? How about her playing and energy level? There isn't much of a difference in her size. Has she been checked for parasites, worms? Has she had blood work? Also how is her poo is it firm or watery? 

Hmm, yeah, she's starting to get that big-eyed look that malnourished kittens get. How's her poop? Does it look like she is absorbing all the nutrients? Has she been thoroughly de-parasitized? I would run bloodwork too.

They did an extensive work up on her and she is fine. She is regularly being de-wormed. We have her on AD mixed with dry to help her gain weight.  One major detail that was left out was 2 weeks after we got her we were given 2 more kittens to take care of, but they passed away suddenly.  After the first kitten passed the 2 left were treated with antibiotics but the second one passed mid course.  Maggie survived but never really showed any signs of illness.  I believe it was Panleuko, but Maggie was vaccinated before the other 2 came in which saved her.  Unfortunately the humane society is so busy they can't afford too much resources on Maggie since she seems so healthy.  I can't take her to our regular vet until she is officially mine. 

You just have to get her to 2 pounds and then she's yours, huh? I'd feed her the highest calorie foods, like a/d and Fancy Feast kitten, and a really high calorie dry food. I suspect she has some kind of enzyme/metabolic disorder, but maybe you can get her fattened up at least a little before figuring it out. It might also help to add some KMR or Pediasure/Ensure to her food.

I would suggest goats milk, raw if you can find and afford.  Otherwise bottled or powdered full fat (ie 3% or slightly more) is OK to use.  Cheaper than kmr too, I think.

Raw egg yolk is good if you dont have problems with salomonella in eggs in your area.  You can mix it with something, for example this goats milk.

Some nice vitamine preparate to throw in cant hurt, as long you dont overdo it.  (it IS possible to overdose vitamine preparates).