Maybe 30s here, but I'm not claiming to be the fastest; I'm wondering if body length is a factor. As a somewhat tall person (189cm) in my 50s I never had any issues changing doona covers.
Had a much shorter girlfriend at one point who simply said "that's not fair" when she saw me do it.
The arm reach factor - I can stretch a queen size duvet tight, holding corner to corner. (AU Queen = 210cm X 210cm).
For fitting, standing up I'll just feed one corner of the doona into the cover and find a corner to pinch, then the same for the other side - then open wide, and shake it out.
It so easy, which is why I was also baffled to see an article about making this easier to do.
(It's a small thing and it doesn't matter at all, but the ex seemed to notice it.)
Had a much shorter girlfriend at one point who simply said "that's not fair" when she saw me do it.