First of all, I think the car example is completely irrelevant. the way you treat your body is nothing compared to a car.
I'll explain how the human body's different than a car, or any other possession.
when years pass, the human body is either growing and building up (from 0yr to ~40). on the other hand, a car just gets old when years pass by. you can in any year fill your car with gas, but you can't in any year smoke (the effects will differ by age, children are sure affected more by smoking than adults). That's one way to put it, and there are many other differences that make the difference between human race and "a car" obvious
in regards to your post, I respectfully disagree.
Age doesn't matter in conversations because when you're having a conversation with someone, you're not talking to the body you're talking to the mind, you're talking to one's beliefs and knowledge, and beliefs change by time and experience, thus, in a conversation, what matters is the time that someone has spent experiencing in the conversation topic, not the age of the person.
Age still matters! Yes, it doesn't matter in conversations, but imagine a 5-year-old making love to a 35-year-old. This is unacceptable under any circumstances. despite the ethical barriers, the 5-year-old body and organs are not prepared for this action. I mentioned before that the human body develops as years pass by, in this case, age matters in any activity that depends on the age of who does it.