when to start co2

this will be my 1st grow with co2. I set the co2 system up while my last grow was almost done flowering. I have heard that small plants do not need as much co2. Right now, the plants have just showed their 2nd set of leaves (not including the cotyledons)

When should i start introducing the co2 to the grow area?

anyone......?

i would start now... of course, only when the lights are on... I used co2 once and it was unbelievable the results...