To decide whether to use numerals or words to refer to a person’s age, follow the advice in sections 2.127 and 2.128 of the MLA Handbook.
In discussions where few numbers appear, spell out ages that can be written in a word or two (2.127). For example:
Johanna came to the United States from Germany in 2011, when she was twenty-eight years old. After living on the East Coast for nearly a decade, she moved to Los Angeles when she was thirty-seven.
Marcia took a cruise to celebrate her seventy-fifth birthday.
Use numerals for ages when more than two words would be needed to spell out the age (2.127):
Born in 1922, Wallace will be 101 in October.
The world’s oldest tortoise is 190 years old.
Use numerals for all numbers, including ages, in discussions where numbers appear frequently (2.128). For instance:
Students who completed the survey ranged in age from 12 to 21. Approximately 50% of those surveyed were between the ages of 14 and 19, roughly 30% were 19 or older, and just under 20% were 13 or younger.
Work Cited
MLA Handbook. 9th ed., Modern Language Association of America, 2021. MLA Handbook Plus, 2021, mlahandbookplus.org/.