What are grow media, nutrients, feed/water sched, ppm/ec, pH?
On my outdoor plants, once they get over 2 or 3 feet tall, they feed heavily. I have a lot of older/lower fan leaves that turn light yellow like that. By the time they get that yellow, they separate from the stem pretty easily, and I pick them off. By the time my plants are larger, like over 5ft tall, I pick off at least one or two from each plant almost every day. I assume it's because they are reusing the nitrogen and/or other stuff from those leaves on new growth. When that starts happening, I bump up the nutes a little, like to full strength on the GH nutes. Right now, i'm feeding the big plants around 1200-1300ppm. It really doesn't look too problematic on yours. The new growth looks healthy. Your plants are at the stage where they are the hungriest they will be for the next 3 to 4 weeks.