Timespan for a generation

What's the average length of a generation in your family tree? The Rootsweb DNA newsgroup has been buzzing with debate on this seemingly simple question. Its not something I'd thought too much about -- I've used 25 years as an approximation as it makes the mental arithmetic easy, while realizing the actual value is probably a bit larger.
By generation we mean the average interval of time between the birth of parents and the birth of their offspring. The newsgroup debate raised some red herrings, including life expectancy which is dominated by child mortality. Some were concerned about trends in age of marriage.
I did the calculation for my own paternal line using the father's date of birth and was surprised to find the generations averaged nearly 38 years! Along that line, but using the mother's birthday, the average per generation was nearly 35 years. My ancestors for several generations were younger sons of large families. The difference isn't too surprising as men tend to marry women a couple of years younger, and can keep fathering children longer, but I was surprised the generation span was so large.
Along my maternal line, using the fathers' birth dates, the generations were 27 years, the mothers' 26 years, much more like my supposition.
To get a good average you need a lot of data. In issue 36 of Your Family Tree Magazine, the April issue, there's a short article on the world's longest genealogy, 86 generations starting with the four times great grandfather of Chinese philosopher Confucius (551 BC - 479 BC). Apparently there are seven direct descendants alive today, and undoubtedly many more who are undocumented. In round figures that's 80 generations in 2,500 years, or about 31 years per generation.
I shall be using 30 years per generation for rough calculations from now on. 25 years is a good rough estimate of the parents age at the birth of the first child.