When plants get older, greens and the like, and get close to bolting (going to seed) they have a tendency to start to get more bitter and when actually bolting they taste terribly bitter. And they actually don't need to be old if photoperiod and/or temperature is off. I've had spinach bolt when it was barely out of seedling stage due to too long of a photoperiod. Could be that or possibly a nute deficiency. Since it's affecting only the older leaves it would probably be a mobile element such as NPK and if you're getting pale older leaves usually N. Or could be a combination of both.