Not really. Not all the Timely stories are in Marvel continuity, by a long shot, and they've been massively altered or retconned by things like Invaders, Liberty Legion, V-Battalion, etc.
The Marvel Age of Comics was a term first used by Stan Lee when laying the foundations of the actual Marvel Comics universe (or leveraging the talents of Kirby and Ditko to do so, depending how you look at it) in the early 60s, starting with Fantastic Four. Timely and Atlas inspired and led to Marvel, but were not really the same thing.
So now the Marvel Universe is Mark Twain?
It doesn't have to be part of "continuity" to still be a part of the Marvel Universe. The characters and stories from 1939-1960 are still Marvel stories, even without the "Marvel" logo in the top lefthand corner.
Not really. Timely or Atlas stories, which may or may not have been ported over into the Marvel Universe once it started being published.
Especially the romance, horror, monster, science fiction and Western stories, which comprised a far larger amount of Atlas' output than superhero tales.
Sure, not all. But by using something, even one thing, from those earlier stories would alter the supposed start date of the Marvel Universe to before 1961.
Hyperbole. Wonderful. Nice chatting with you.
Just pointing out that the chronology of the Marvel Universe is internally always going to extend before the date at which it first started getting published in ours, so just because a story references events in 1939, doesn't mean that all Timely stories published from 1939 are in unless shown otherwise.
Considering how little they reference it at all now, and that even when they do they tend not to term it that way (this week's Mighty Avengers being an exception), I doubt they'll feel the need to bother doing so explicitly.
And drug use implies a decision to take a drug, which being exposed to the drug via an explosion is definitely not. If you are standing downwind when somebody throws a bunch of pot on a bonfire, that wouldn't make you a drug user, just somebody who was exposed to marijuana. Likewise, Cap has no drug use to retcon, just drug exposure. If there had ever been a scene where he voluntarily chose to take or expose himself to more of the drug, you would have a point, but since there wasn't, you don't. Take this reboot as an opportunity to drop it once and for all.