If you have a hole in your pocket the penny drops a lot.

Yes it is old fashioned. More examples: I wondered where I had heard the song on the radio before and then the penny dropped! I actually had the CD. It takes some students quite a while to understand algebra and then one day, the penny drops!

It's still used today =] When the penny drops, someone belatedly understands something that everyone else has long since understood. Example - I sat and thought for a moment and then the penny dropped.