Buddy, I wouldn't worry. Leaf yellowing on older and lower plant growth is totally normal! As long as your new growth is healthy and green you're ok. The yellowing is due to the lower portion of the plant not getting as much light as the upper and would also be the first food stores to be used up.
The same thing will happen towards the end of the plants life (harvest time) when you come to flushing. Only difference is it will be the whole plant, but its a good sign. You're on track doing fine. If you need any more info don't hesitate to drop me a message. Hope this helps. 😉