The 18 year old vote was in 71. Draft ended and draw down of US troops began in 73. As usual, many things coincided to bring an end to the demonstrations.
The draft lottery had a quieting effect as well. You knew early on your chance getting picked or not. I was in that first lottery, 1969.
Nixon began planning for ‘Vietnamization’ about then as well, turning more of the combat role over to ARVN soldiers, which also reduced the number of Americans subject to being drafted.