It can be caused by an array of issues relating to nutrients, temperature, moisture, and pest damage. This time of year the hot weather is curling the top leaves on most of my peppers. Make sure not to over water them and let them grow, the leaf curl should sort itself out.

-Phil

Yep, relatively young ones there. My peppers went through various stages of leaf maladies. They always seemed to snap out of them or I just realized it was a natural condition that didn't warrant any alarm. There may be a real problem with your plants but at this point, I wouldn't do anything drastic to try to fix the issue. Sometimes, you can create a worse issue. Since my plants have gotten older and have fruit on them, I have become much less alarmed when a change takes place in my plants. Probably better for me and my plants that way. I would keep an eye on them and in the mean time only change obvious things. Their color looks fine and they don't seem to be wilting.