Margarita, I am not sure if there is a precise answer, but we normally use the word medium with size or physical measurment and middle on a scale of reference. For example, we wear a medium sized shirt, a person is medium height, medium range (distance) aircraft. Someone is middle aged or mid-career which refers to a time range/scale. I hope this helps :)

That's a good question, Middle refers to a place hence why we say "IN the middle". Time is distance / speed = Age is literally the time we spent on earth roaming the sun :) Medium refers to size or difficulty I'd like a medium shirt The intensity of the workout is medium