The main reasons we withhold dairy in infancy are 1. because of potential allergy to casein, the protein in milk, and 2. to avoid displacing breast milk as the main source of nutrition.
Butter is more than 80% milkfat, and contains almost no casein, and therefore I wouldn't worry a hoot about #1. #2 is only a concern if you're going to offer HUGE amounts of the stuff.
So I'd offer butter at any time, to any baby who's otherwise old enough to eat solid foods. I'd offer good organic butter, and if I could find pastured butter (from grass-fed cows) I'd offer that by preference, because it's richer in fat-soluble vitamins and essential fatty acids than conventional butter is.