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...the old stuff, not the new stuff (although that's good in its own right!).
It's not that hard a concept, it was intended as a kids' show, and the shoestring-snapped budget FX will make everyone giggle (we used that as a jumping-off point for the concept of "suspension of disbelief"). Quite a few episodes are now available on DVD.
Six and up normally, but my two-year-old stops in her tracks and looks at the TV every time she hears the theme music, and she cheers every time the Doctor is onscreen (and that goes for all of the incarnations of the character she's seen).
Furthermore, for car trips, download some of Big Finish's Doctor Who audios to your mp3 player from their website. Hey presto! Radio drama!