Hi! I met this question yesterday，but I couldn't remember the whole sentence. Its probable structure is

______Mary became the first woman who completed some achievement

For the blank，there are 2 options which I am very confused about
A. At the age of 40
B. Of 40-year-old

I don't remember which option I chose at the end，but I guess I took B 😥😥

Question

Hi! I met this question yesterday，but I couldn't remember the whole sentence.  Its probable structure is

______Mary became the first  woman who  completed some achievement

For the blank，there are 2 options which I am very confused about 
A.  At the age of 40
B.  Of 40-year-old

I don't remember which option I chose at the end，but I guess I took B 😥😥

Answer

"At the age of 40, Mary became the first woman who..." is correct.

"Of 40-year-old Mary became the first woman who..." is incorrect.

However, the problem is the word "of." It would be correct to say
"40-year-old Mary became the first woman who..."

"At the age of 40, Mary..." is a statement about time. It is similar to saying
"In 1983, Sally Ride became the first American woman in space."

In "40-year-old Mary became..." is a statement about a characteristic or property of Mary. "40-year-old" is functioning as an adjective. It is similar to saying
"86-pound Olga Korbut became the first woman to perform a backflip-to-catch on the uneven bars."

Answer

Sorry to say this though the correct option is A.

At the age of means: When she turned 40.

Answer

In what caae，a sentence would start from “Of+specific age”？😥