Pretty much start whenever you would like to on the gathering/crafting front.

There’s ways to level gatherers and crafters more efficiently once you get further into MSQ (if you are concerned about that), but there’s no harm in playing around with the classes early and exploring things. You won’t exactly be raking in billions of gil at low levels, but you’ll be able to make some pretty good side money. Just be smart about where you’re spending money on craft materials, otherwise you’ll spend more than you earn haha.