Re: How old?
I think you make some good points, as I think it's a number of things, but mostly pride in what you drive, if it's yours or your parents. If It's a car you really wanted, and you got it, I think you will take care of it. There are people who are car people, and others that just see it as transportation, especially if it's dad's or mom's car. There's also the point of did you buy it, or did dad give it to you (and there's nothing wrong with that). If you are a car person, you will keep it clean.
I've always kept my car clean. Even my first family car hand-me-down when I was a kid.