It's hard to believe that a woman her age could accomplish so much.

My answer:
no change

Why not this?

It's hard to believe that she could accomplish so much.

I believe this is a sentence in a "non-sexist, non-agist" type of assignment, so you need to get rid of references to male or female and to age.

To remove gender and age references from the sentence while keeping the same meaning, you can rephrase it as: "It's impressive to see someone accomplish so much." By removing the specific mention of a woman and her age, the focus shifts to the general achievement without any bias related to gender or age.