I've never had a problem with cream of tarter being old, I use it with baking soda as a replacement for baking powder (mix them right before adding to the flour). It gets super clumpy if it's damp though, which can be an issue, but it doesn't lose it's acidity unless it's mixed with an alkaline during storage.
But it is super cheap, so maybe you should just replace it.

I believe cream of tartar has an indefinite shelf life if it has been kept away from heat and air. I have never had any problem with aged cream of tartar. As Antonia James notes, cream of tartar is one of the constituent ingredients of double-acting baking powder. I am sure Shirley Corriher and/or Harold McGee can explain in more detail, but alas, I cannot put my hands on my copies of their books right now.

I think you mean that cream of tartar is expensive not inexpensive? I favor buttermilk biscuits too.I though C of T had an indefinate shelf life, maybe a bad recipe?

Mine doesn't have an expiration date, either! I'm going to start marking my purchase date with a Sharpie. But I'm also going to contact the manufacturer to follow up on this. I had some that had been around at least a couple of years. I used it in a (food52) recipe which called for it, in place of the baking powder that would typically be in recipes of that sort. It was horrid! (The baked item, that is.) I assumed it was due to the expired cream of tartar, but of course I'll never know. I urgently needed the item, so I had to make it again, so I made it using a conventional recipe that calls for baking powder; it was such a nuisance. I'm not taking any chances on it again. It's not worth the risk, especially given how inexpensive cream of tartar is. Also, if you are making biscuits, and you don't have time to go out and buy more cream of tartar, use baking powder. One teaspoon of baking powder is equivalent to 1/4 teaspoon of baking soda + 1/2 teaspoon cream of tartar. You can also go totally old-school on this, not using baking powder at all, and instead just use 1/4 teaspoon baking soda + 1/2 cup buttermilk, decreasing whatever liquid is called for by 1/2 cup. Buttermilk biscuits are excellent. Good luck!!

Just how old is it? I don't think mine has an expiration date on it, but I know I've had it for *years* and it works fine. As long as it's been kept dry (shouldn't be clumpy) I think you should be all set.