Of or involving the cells of an organization or movement: The assessment of opposition to any totalitarian regime ... is notoriously difficult, for any effective movement must be secretive and cellular (Anthony Sampson). 4. Of or relating to a cellular telephone.
In the past, a human generation has been considered to be 20-25 years. Older genealogical documents will use these numbers. However, genealogical studies have revealed that this is too simplistic and can result in large inaccuracies during statistical calculation.
Short Answer: 25 years, but a generation ago it was 20 years. Long answer: It depends on what you mean by generation.