How do you define the young segment - that is to say what does young mean for marketers - from which age and until when are customers considered young?

It varies depending on the product being marketed. When referring to music, clothes, cosmetics etc. the market targets a consumer at the average age of 15 to 25. Usually any product has 4 stages, infant, growth, maturity and decline, we say young segment when it reflect high return to the company and that's happened in the growth stage.