If it was my own homemade pizza-- as soon as they are physically able to manage it. So maybe seven or eight months? Mine is only soaked whole wheat flour, olive oil, salt, and water, so there's nothing worrisome in there, to me-- although some folks avoid grains in the first year, I don't. I go for whole grains that have been soaked or fermented to be more digestible.
But commercial pizza? It depends on where it's from. I have an ingredient list from my favorite pizza parlor, because of DS's allergies. They obviously don't soak their flour, and it's white flour, but there's nothing objectionable in there. So maybe I'd wait a few more months, but maybe not. Let's say 9 or 10 months???
If I didn't know what was in it, like if it was pizza at a party at someone's house, or if it was something like Papa Johns or Dominos that has who-knows-what in it? I might try and stall baby, and offer something else, but by around 18 months I wouldn't stop them, unless there were obvious issues like an allergy.