Whens a good time to use yeast.

I decided to hold off on the co2 tank until tax return comes. i want to use yeats for right now. my plants are about 11 days old . should i start them out on the yeast co2 right now?

i dont know if it made a differencce but we used it from day 1

i would think co2 doesnt matter until you get into the flower stages

Wait till you begin flowering as you will just waste a crap load of money changing out yeast. Unless you are making beer, wine or a sourdough bread starter I would not even bother as you will find yourself replacing the yeast every week or so to keep the smell of rotting yeast on the DL
Now when they are flowering I highly recomend you get some C02 in there as I have seen the differnce between using C02 versues not using C02 and the yield was increased by an avarage of 25% to 60% versues the plants that did not recieve increased levels of C02 on a previous grow I did.
Somewhere i even have pics of that grow it was over ten years ago. I'll look around a see if I can find them and post them so you can see for yourself.

I also started my little c02 setup from the first seedling. This is my first crop, so I have nothing to compare it to, but I figured it can't hurt. plants all love c02 especially pot.

oh, my setuop costs $5 each time and lasts for 3-4 weeks

just started the yeast jug last night. 3 cups sugar, 1/2 gal water, 1/2 pack yeast about