Sensei is used in modern Japan to refer to teachers in educational institutions, in classical arts and crafts, in taiko drum clubs, paper airplane clubs, medical doctors, professors, martial arts, dance, music. . Although the literal term does signify that the person has some kind of age and maturity, there is no set age marker as to who can be called a sensei. I once received a letter from an American martial arts supplier who refused to put an ad in my magazine because he firmly believed that no one below his age level should have the right to be called sensei. He buttressed his opinion by what he considered his incredible grasp of Japanese culture, although his Japanese language and cultural abilities rarely went beyond his dojo doors. Rather than buckle under to a potential advertiser, I sent him a note saying that we weren't going to give up our own values just to try to get him as a paying advertiser.
Some systems also grant the titles of renshi, kyoshi and hanshi .
These are derived from kyudo and academic degrees, and may be defined as a sort of assistant professor, full professor, and tenured big-cheese professor. These are special teaching titles, like a Ph.D., and are usually not used in addressing someone in the usual dojo conversation. Thus, someone with a kyoshi rank is usually simply referred to, for example, as Smith sensei, no matter his teaching license level or belt rank, unless you are discussing that teacher in a biographical paper, introduction, or so on. They are usually bestowed in terms of teaching ability and experience. Thus, you could write about someone, listing his credentials as Joe Blow, hanshi, godan, as you would address someone in a formal text as Mr. Joe Blow, Ph.D. in Economics.