Bottom set of fan leaves yellowing is normal after that long for multiple reasons
Bottom leaves are not getting as much light, becoming less useful.
Also the simple truth is that younger leaves are more efficient at changing sunlight into carbohydrates that can be used by the plant.
If you have 2 leaves of the same size, one old and one new, the new one can "photosythesize" better than the old one. (Produce more food with less light)
In other words, old, lower level leaves aren't as important to the plant anymore and are naturally yellowing, and will eventually fall off... there is no reason to worry about it at this point.