I indicated source information for Chapter 1 of Smartcuts in the End Notes section of the book; however, the Presidents calculations were put together from a variety of sources, and I felt that it would be easier to share the data tables themselves than to spell them out in print.
*Note that everything in the book has been confirmed by one or more independent fact checkers (Chapter 1 had two fact checkers go over everything). It is certain that exact figures of averages will shift as new people are elected. But the pattern of presidential ladder-hacking and dues-less presidents outperforming stubborn long-time politicians will, I expect, not change.
Here are two spreadsheets compiled from publicly available data about presidents' histories:
The years in office shown in the charts in Smartcuts display years spent in federal elected office, as stated in the book and end notes, unless otherwise noted. City or state offices (other than governorships) and political appointments like secretaryships are not seen as indicators that one is on the ladder to realistically becoming president, so they were not included. However, the lateral-thinking/working patterns still hold when you include those aspects in the presidents' journeys. The larger point is that the best presidents got there nontraditionally, and the worst presidents were the ones who got there by paying dues.
If you're interested in drilling into more research on presidents' ages and journeys, here are a few helpful links:
Average ages of congress by position throughout the years: http://www.lifecourse.com/rdb/indicators/age.php
A quick glance list of other offices held by presidents is kept updated by the folks at Wikipedia:
Explore the losing candidates' data here: http://www.loc.gov/rr/print/list/060_ran_chron.html
And this is a fun infographic on congressional ages and parties over the years: