Because the title of "professor" is hard to come by! It is a symbol of a person's academic level. Generally speaking, these people have been engaged in a certain job for more than a few decades after graduation, and have made certain achievements, published academic papers of their major, which have been recognized by the academic community. Only in this way can he become a professor, and he has invested a lot of time in the direction of becoming a professor. So generally speaking, those professors are old.