your choice, you can either go 24/0 or 18/6. lotsa arguments wether 24hrs of light will benifit you or not, im a strong believer of give them the 6 hour rest, thats what i take every night, why not give it to them?
but when i mean how old because one guide i read said when seedlings give them the 24/0 and when u start to veg use 18/6 when do i determine when they aren't seedlings anymore and when to switch the lichts
A seedling does not need 24/0, Like NaughtyDraed said just stick to one light schedule why make it harder if ya dont need too.