The average age of a car in the US was 11,9 years in 2020.
https://www.statista.com/statistics/...projected-age/
The age has been increasing slowly by about 0,1 years every year.
So I'd assume the 50% mark will be reached in the late 2030s/early 2040s if EV sales increase further.
50% sales should be reached by mid/late 2020s as manufacturers are finaly releasing the kinds of vehicles average consumers would buy.
That's just a guess, don't take it too seriously.