Thinking about the math a little bit ...
Suppose we deliver 200MM doses into people's arms by the 100th day of this new administration, that's roughly 100 million Americans fully vaccinated. Chances are there would be more because the single-dose vaccines are increasingly being used more.
The "100th day" brings us to April 30. The "100 million Americans" figure is roughly the number of all Americans 55 and older. Take out the anti-vaxxers and you can conceivably push that line down to "50 and older."
Once you get to that 50~55 line, it is quite conceivable that you can open eligibility to "all adults."