A picture would really help out but off the top of my head there is one reason for the bottom leaves to yellow and that is because its hungry and pulling all the stores out of the lower leaves. Why its hungry could be dozen different things that we  would need more info on to figure it out.

Is it 7 weeks from seed? What are you growing it is, soil, coco or water? What have you been feeding it and how much. Whats your PH levels, etc.

This flow chart should get you there but providing more info is a big help