If she’s old enough and competent to drive a multi-ton moving vehicle capable of great harm and damage, she should certainly be ready for marriage. Back in the good ‘ole days, and not all too long ago at that, we used to get married not long after bar/bas mitzvah. One should surely not wait long after high school graduation to move on to the next phase in life.
In any event, the girl referred to in the OP is at least 23 by now and hopefully has received several mazal tovs already.