I don't like posts like this because they ignore the nuances of reality, and I'm not talking about some complicated nuances either. These are things we have all observed, including yourself I'm sure, and have grown to treat them as facts, and rightfully so. Yeah, sure, your above argument does sound like a rebuttal in theory, but it literally ignores the hundreds of matches we've watched Federer play for 5 years between 2010 and 2015, and last I checked, it wasn't just Djokovic and Nadal he was losing to. His results (and level of play) dropped considerably against everyone. Yes, every now and then, he'd play great over a period of a week or two and roll back the years, but for the most part, it was widely accepted that he had gotten old and declined. So, I ask you, his rejuvenation in 2017 invalidates that how exactly? Even the "fountain of youth" argument doesn't hold up, because if he found it, then he found it in 2017, not before.
So yeah, it sounds logical to ask "how come Federer was too old when he was younger?" but we all watched tennis extensively over the past half a decade and we all know the answer to that. He was too old because he was in his mid thirties, playing poorly, suffering with injuries, moving slower, clearly struggling to adapt his game to his physical "limitations" (of course I use that term very lightly), etc...
In 2017, he was fresher, he adapted, he was sharper, he served better, he hit crisper, he played smarter, and he took lots and lots of time off before the year and throughout it. Hope that answers your question.