Does a renewed retirement extension in Thailand start from the expiry date or the application date?

yearly extensions are ADDED to the expiration date you have now, so applying early isn't gonna change the date of your expiration

I have had the same expiration date for 14 years now (my first non-O visa expired April 5th and that has always been my yearly expiration date.

In bangkok you can apply up to 45 days early, here's where I applied 42 days before my current extension ran out and I got stamped until the normal April 5th date like always