Hey, let's not forget this: MANY people not only forget to maintain the inside of their cars, but also forget to maintain the outside. If your car isn't washed regularly, it'll look like crap in a few years.
The worst thing you can do to a paint job is just let it get dirty. Letting the dirt pile up not only stains the paint, but starts to destroy the protective clear-coat, too!
I just washed, waxed, and polished my 13 year-old Maxima. (And applied some Armor-all... love that stuff.)
And my Maxima looks *$%&ing awesome, considering how old it is. It looks newer than most 5-year-old cars I see on the road, these days.