Can I apply for an extension of stay based on retirement before I turn 50?

49 years and 360 days isn't 50

Nope you cannot,

That extension is issued for the real reason of "กรณีใช้ชีวิตในบั้นปลาย" <- which says in the instance of living out the end of your life 😮 (now you see why they call it "retirement" in english 😛 )

and

there is a clause that says มีอายุเกิน 50 ปีขึ้นไป <- be 50 years or older

They won't let you switch to that extension until you hit 50

Now you say you're on a yearly extension based on marriage to a thai national, so you might ask them if you could get the 60 day visit thai family extension and that could "buy you the time" until you turn 50 so you could apply for the year extension based on retirement after that 60 days winds down