I used to chaperone my sister and her boyfriend (now husband) all the time. I was 16 at the time, she was 25. I always left them alone, because chaperoning was boring. I would just go into my bedroom to read or watch TV or whatever and leave them in the living room. My sister told me I was the best chaperone ever lol.
When I was dating my ex-husband, my mother let us go out alone quite a bit, which is weird since she's a very strict JW.