Can I still work at Uniqlo at the age of 40?

No.

After checking the information on the boss direct recruitment official website, Uniqlo’s recruitment age range is from 18 to 35 years old, and 40 years old does not meet the age requirements for recruitment.

Uniqlo is the core brand of Japan's Fast Retailing Company. It was founded in 1984 as a small clothing store selling suits. Now it is a household name.