That's not entirely true. If I buy a used car, maybe the previous owner didn't take care of it, but that doesn't mean I shouldn't either. It still has a better chance if I regularly do the maintenance it needs.
I was shit at taking care of my teeth when I was a teen/early adult. As a result, I have like 4 crowns on my back teeth, and have had numerous fillings, etc. But it wasn't too late to change my habits. Just because I didn't start from a child, I began a better teeth brushing/flossing/gum care routine and have not had a cavity in probably 10 years. By regularly attending my cleanings and following a healthy routine, I saved my self thousands of dollars.
I've had medical issues that I've put off in the past that have cost me more simply by not fixing something when it was initially wrong. And by not checking to see if things were wrong earlier on.