Truet Cathy: 56 when he opened his first Chick-Fil-A
David Didau: b. 1972 (ish), started his blog in 2011 at the age of 39.
Richard Foster: b. 1942, spent career as pastor. Dallas Willard was a parishioner of his. His Celebration of Disciplines came out in 1978 — he was 36. (Wow.)
E. D. Hirsch: 59 when his first big book came out
Tim Keller: 39 when he started Redeemer, 58 when his first bestseller was released Reason for God.
Doug Lemov: b. 1967. 42 when Teach Like a Champion came out.
Mike Schmoker: 39 when he wrote Total Quality Education, 60 when he wrote Focus. Now, at 39, Mike was actually a research analyst at Amphitheater Schools in Tucson, AZ, and he co-wrote the book with his superintendent at the same place.
One thing you notice in the book's first words is that Mike has always been good in communicating a common sense urgency: “Without waiting another day, we now have the means to begin improving our schools… .” That line is so Mike — we can do this thing that's super hard; we have everything we need, right now!
Additionally, in TQM and in the notes on the author in that book, you see that Mike was laser-focused on something called Deming's total quality management. He lists the articles he wrote about that, and the mountaintop experience of presenting with W. Edwards Deming. So picture Mike in his thirties buried in and analyzing research, selecting a master's work to emulate, and through that work of emulation and application becoming the Mike Schmoker we know.
Dallas Willard: b. 1935, d. 2013 (age 77). His first work on Christianity was Spirit of the Disciplines, 1988 (age 53). His breakout work was ten years later — Divine Conspiracy, 1998 (age 63). This book won the Christianity Today book of the year award. After this he published quite regularly.
Willard was married at age 20, and he had two children and a grandchild. His career was as a philosophy professor — Christian writing it seems was something he saved for the later chapters of his life.
Dan Willingham: b. 1961. 49 when his first articles appeared in American Educator.