“The transition to Medicare for All would occur in two years. One year after the date of enactment, persons over the age of 55 and under the age of 19 would be eligible for the program. Two years after the date of enactment, all people living in the U.S. would be eligible for the program.”
Source: Medicare for All Cacus, “Medicare for All Act of 2019”, Summary.
Full text of H.R. 1384: https://www.congress.gov/bill/116th-congress/house-bill/1384/text