Quote Originally Posted by Math Demon View Post
OK, Tthree, I'll bite!!

Why is the first dog year equivalent to 12 human years; and the rest, 4 years? Is it comparing to human capabilities? mammalian capabilities? Just curious.
You may be barking up the wrong tree.

I always assumed it had to do with reaching physical young adulthood.

WebMD:
1st year = 15 years
2nd year = 9 years
3rd+ years = 4 years for small dogs but accelerate aging in late years for medium and large breeds (5.5 years).

To answer your query it is based on the rate of maturity for humans vs dogs at hitting various points where parallel changes are expected. Sorry about the inaccurate memory. It has been decades since I read tis. I thought it interesting enough to remember but didn't get the details quite right.