They are basing her appearance on Kathryn Hahn, who is about 50. I see why they chose Hahn.
A 90 year old doesn’t have the physical strength to do those stunts and comic book Agatha looks like she is 100. Sure, you do have good older actors, but there certain things like stunt work you won’t see them doing over a certain age.
There are actors who get themselves into good shape in their 60s or even 70s to do physical roles, but Agatha often like over 90.
But whatever, I love her and happy she appears more frequently these days.