My plants started to do the same thing at that same size same leaves everything. I was worried about it but I kept feeding them fish emulsion like once a week and the yellowing never spread. Being a newbie grower and doing it all on my own before discovering these sites I am surprised that I got 5 out of 6 ratio female/male and have 5ft monsters still in veg. I know I made plenty of mistakes one being no holes in my starter pots I'm sure there was plenty of salt build up. MJ I have come to learn is an amazing plant. I had one completely flooded after a long rainstorm all the leaves died but regrew after i transplanted it into a place with more drainage. 2weeks ago it was 3ft shorter then the others. not anymore it caught right up. Guess what I am saying is I wouldn't worry about that little yellowing on the old leaves to much, just give it some good organic fert about once a week she will pull through.