the older leaves will start yellowing some as it pulls it out of the old foliage and puts it into the new stuff. when you do start nutes you need to be very careful, cannabis nuteburns pretty easy at first, start at 1/4 strength and kind of slowly work your way up.if your leaves get really dark green and/or develop orangish colored dead spot on the edges/tips it means its getting close to burning and you need to back off on the feeding. after thinking about it a little bit heres what i would probally do-start light feeding at about 4 weeks(maybe sooner if the plant looks like it needs it)give it like 25% strength of the nute and see how it reacts, if it shows no problems you can keep lightly feeding every 3 waterings or so, just keep your eyes open for dark green leaves and burnt looking orange splotches/tips/edges. if you see those stop feeding and depending on the severity you might have to flush the plants.