Is it common to express interest in post-doc positions well before you are qualified to apply?
One semester into your PhD is probably a little too early to start contacting potential post doc advisors and future employers—the crystal ball is way too fuzzy, and their plans almost certainly aren’t known. (They may not even be doing the same area four or five years from now!)

You could certainly contact them about your interest in your research and introduce yourself, but I would wait until you’re slightly further along to start discussing future jobs; I’d say serious discussions about that are appropriate when you’re about two years out from graduation.