How long is a generation, you ask?
Short Answer: 25 years, but a generation ago it was 20 years.
Long answer: It depends on what you mean by generation.
A Biological Generation
…is simply the unscaled transition from one parent to one offspring. In humans, the Biological generation does not have a standard length but there are limits. So you are in one generation, your mother the previous, your child the next one after you, etc. regardless of when any of you were born. As long as your Uncle Willard does not marry your Sister Betty Jean, this is not complicated; This is what people often mean when they use the term “generation” but not what they mean when they ask the question “how long is a generation.”
From:
http://scienceblogs.com/gregladen/2011/03/01/how-long-is-a-generation/