My kitty was an indoor kitty for most of her life -- she never went outside without a leash on, and then only in the back yard (we strung up a kitty-run so that the kitties could have full access to the back yard, but not get out of it).

One day she saw our outdoor cat leaving out the front door, and made a dash for it! Zoom! She got about halfway down the front walk and froze... what was all this stuff??!? Then a car went past -- and ZOOM! back into the house. We couldn't get her to come out from under the bed for about three hours.

When my mom moved in with my stepdad, when she was about 10 (my kitty, not my mom!), they lived in a very quiet, kitty-safe neighborhood, so they let her go out. She hated the cat flap (although sometimes she'd brave it to get to sit on a warm car!) but when they let the other kitties out the front door, she'd sometimes follow... never got further than the front porch, though. :)