I find catnip a short-lived perennial with sometimes almost more of a biennial life-cycle. I let the mature plants self-sow and move the young volunteers around to keep catnip going here.

If you're going to start seed indoors with the idea of moving it outdoors, I would probably wait until it is time to start peppers for the summer veggie garden. Plants started to far ahead of the normal growing season tend to be rather spindly and weak for people. Try to time it for hardening it off outdoors when your location has warmed enough.

FataMorgana

By now my advice won't find usefulness with you, but another is bound to read it and find it useful. Catnip seed often take three to four weeks to germinate. There is a way to shorten time to germination to as little as one week. My way of doing it is somewhat distasteful. Here in the southeastern U.S. no one would even pay attention to it. It works very well. Fill the pot section of a ceramic toilet with potting soil. Place the toilet in the yard where the sun's light will strike it all day long. Pour the seed on the soil. Wet the soil. Lower the toilet's seat cover so that the toilet is bowl is covered. One week, maybe a day or two less, and catnip seed will be sprouting.