I got my first, 1 year, Thai licenses ( motorcycle & car ) on the 28th December 2010 - i came to Thailand with an International Driving License ( IDP - issued by the U.K. post Office ).
My Birthday is the 25th December. As the OP says, the effective date of the license is the 1st birthday after it is issued, so my license was valid until 25 Dec 2012.
I went to renew them both ( upgrade to a 5 year ) in early December 2012, only to be told that it couldn't be renewed until it had expired, so i went back after Christmas, and got them both renewed / upgraded to 5 years.
So, 2017 comes around and I wait until after my birthday to go top renew - only to be told that they've changed the rules ! I should have renewed within the last 3 months of the license's validity. Because I had let the license expire, I had to go through the full gambit of tests, video presentations etc, etc. to get them renewed. They now expire on the 25th December 2022.
So, I suggest you check with somebody in the know; if in doubt, go to renew early, rather than late.
Best of luck
The Proffesor
P.S. I'm at an age where my memory is failing, so excuse me if I've got the dates wrong. I do remember that my birthday is at Christmas - I think . . .