Rapid Weight Loss and Weakness in Gerbil

Question
I have two gerbils.  They are both 4 years old, which as I understand is about as long as they are expected to live.  In the past couple of weeks I've noticed that one of them rarely closed her eyes.  The two gerbils would be sleeping together, and when she doesn't have her head buried, her eyes were usually open, if not just a little.  I thought it was strange, but I wasn't too worried.

Yesterday, after not paying much attention to them for a couple of days other than keeping their bedding, food and water in check, I noticed something wrong with the one who I just mentioned with the sleeping issues.  She was very weak and tipping over, such as trying to balance in the food bowl.  She also looked and felt like she lost a lot of weight.  I don't want to say that she lost almost half her body weight, but it does seem like that.  She was never an overweight gerbil to begin with.

I havn't seen her drink recently, but it's one of the bottles where they have to stand up slightly to get a drink.  They have been fine with it for 4 years now.  I have been meaning to get a better one, especially now that I don't think this gerbil will have the strength to drink this way.  She does continue to eat, which makes this rapid weight loss and weakness even more strange.  I'm not certain of the type of gerbil, but they are very small.  The other gerbil is just fine, being a little chubbier that she is; she still runs in the wheel, sleeps normally, eats and drinks.  It's this other one that I'm very concerned about.  I don't think she'll last much longer in her current state.

Answer
It sounds like she's "wasting away" due to old age. There isn't much you can really do though. It really is a part of old age. You can try and take your gerbil to a vet to get looked at and maybe some antibiotics would help her. I hope she gets better.