The leaves curling are most likely a sign of stress. The trick is figuring out what is causing it.

It's very easy to overwater when they're that small. It may be dry on the top, but, may never dry out on the bottom. Be careful of that.

Cool temps can cause growth to slow way down as well.

Window light isn't as strong as you might think with newer more efficient windows. I've kept plenty of plants going in the window over the winter, but, they grow more slowly and not as sturdy / strong as directly under floro tubes or outdoors.

If I were to guess as to the cause of the leaf curl, I'd say temps or maybe too much water.

Thank you for the follow-up. Could it be temps as in too hot? I realized that it gets hot by the window with not much ventilation (they sit behind the couch). It probably gets about 80-90 degrees or so back there.

I was thinking more cool temps, so, that may not be it at all. I can't imagine it being from too much heat / light indoors. 80-90 sounds perfect, really.

Not too hot. Peppers LOVE high temps up into the 90s. I'd way too wet and not enough sun.

FWIW, many superhots row very slowly at first. You would do better to plant a dozen and take the strongest rather than invest your hopes on just one.

Dennis