It's normal for the oldest leaves to die eventually, but the yellow tips mean the plants need fertilizer. What are you using?

You can either add a slow release to the potting mix or add a small amount of instantly dissolvable fertilizer to the watering can.

Ecotypes that take longer to bolt (like Col-0 and Ler) will need more fertilizer than ones that bolt after just 5-6 leaves (ws).

Katie A S Burnette Thank you for your answer. I'm using liquid fertilizer with micronutrients. I even thought I might be using too much of it. Could this leaf yellowing be due to another factor? Also, I'd like to know what type of fertilizer you use.