Jalapenos will actually turn red when they are fully ripe. However, people often use them when they are green. They can get what is called "corking" on their skin and that is also an indicator of when they are ready to harvest when green.

As has been mentioned, when they are ripe they turn red. I use them when they are at a size that I think is worth using, LOL, usually about the size of my thumb at the smallest. A combination of the red and the green makes a lovely jar when sliced and pickled, but my husband mostly just slices them up and eats them.

Annie

And not all mature jalapenos get corky.

From my experience, the ones with corking are hotter than the ones without.

When I grow them, I let them turn red before I pick them. If I am desperate, then I will pick some when they are green.

IMO the nice thing about peppers is that you can pick them at any point in the growing season. As mentioned the longer they are on the plant the hotter they will be.

FWIW, I read that in some cultures, those splits make the peppers more desirable, and therefore, more expensive. I believe it's because they're considered to be more flavorful.