I didn't write or mean to imply the quote about improving at one age being easier at another age. It's all a matter of consistency over a period of years, and doesn't really matter what age you start at (within reason). Give anyone three to five consistent training years, and they will improve to within several percent of their total potential.
The form argument sounds specious to me. Michael Johnson had the worst form possible, from a technical standpoint. I suppose you would have corrected his Barco-lounger lean and he would have run sub-43, right? Right back at you, Sprint Geezer: where is the proof? Show me a SINGLE, DOCUMENTED case of a late-career sprinter being coached to 4% gains due to form and mechanics modifications. It doesn't happen, and it's not because sprinters are uncoachable egoists. It's because mechanics matter less than talent and consistent training.
Back to the matter at hand. Carmelita is achieving her consistent training by using artifical means to increase her durability. Drugs don't make you fast; talent will do that. Drugs don't make you work harder; they allow you to work harder, and they prevent you from getting injured under massive, successive training efforts. I stand by my analysis. It is impossible for someone who is clean to achieve the kind of progressive improvement shown by Carmelita Jeter.