For what's is worth (since this topic comes up frequently) here are, in simple terms, the primary teachings the Society has used to define the "Generation".
1897 – A group of contemporary people
“ Other uses of this Greek word (genea) prove that it is not used with the significance of race, but in reference to people living contemporaneously.” – Studies in the Scriptures Vol. 4, p. 602-03,
1927 – The anointed
“The irresistible conclusion therefore is that Jesus referred to the new creation when he said: “This generation shall not pass until all these things be fulfilled.” This then would be a strong indication that some members of the new creation will be on the earth at the time of Armageddon.” – Watchtower, February 15, 1927
1951 – A fixed period of time from 1914
"The length of time is indicated by him when he said, "Truly I say to you that this generation will by no means pass away until all these things occur." (Matt. 24:34) The actual meaning of these words is, beyond question, that which takes a "generation" in the ordinary sense, as at Mark 8:12 and Acts 13:36, or for those who are living at the given period." – Watchtower, July 1, 1951 p.404
1995 – A group of contemporary people
“…Jesus was not departing from his established use of the term “this generation,” which he consistently applied to the contemporary masses with their “blind guides” who together made up the Jewish nation.” – Watchtower, November 1, 1995
2008 – The anointed
As a class, these anointed ones make up the modern-day "generation" of contemporaries that will not pass away "until all these things occur." This suggests that some who are Christ's anointed brothers will still be alive on earth when the foretold great tribulation begins. – Watchtower, February 15, 2008