Do younger Japanese people really use 電気をつける to mean "turn on the light"? It sounds like something that your Grandad would say in the times when electricity was new and exciting. I know it's taught in textbooks but they can have a tendency to be somewhat anachronistic. Is it the most common phrase, and if not what should I be saying?
Yes, it's the most normal way to say it, among young people too.
電気をつける - Turn on the lights
電気を消す - Turn off the lights
If you don't like it then 明かりをつける（消す） is another phrase you could use...