Great question sir, picture a family as that is what Jesus was comparing it to. We believe Jesus received the crown in 1914, thus we have the starting point. We also believe it is speaking about anointed Christians, to those whom the covenant was made.
So all the anointed brothers of Christ alive upon Jesus being crowned as King would be the parents of the generation, the end of the previous generation. Anyone who became anointed Christians during their lifetime would be the generation Jesus was speaking about. Before these die off, the great tribulation will occur.
That is the simple explanation. This video explains it very well to a greater degree if you are interested:
https://www.jw.org/en/library/videos/#en/mediaitems/StudioMonthlyPrograms/pub-jwb_201509_1_VIDEO