what is the formal way of writing numbers? He is 16 years old or he is sixteen years old.

Question

rogue · Answer

Spelling it out in this case would be formal.

anonymous · Answer

He is sixteen-years-old, I think the "-" are used...

anonymous · Answer

There would only be a das between "years" and "old".  The generally agreed upon "formal" method of writing that sentence would be,
"He is sixteen years-old."

jagatuba · Answer

Only use hyphens when the age is being used as an adjective:
Quit acting like a ten-year-old

Here are the rules for writing ages:

Express ages in figures (including 1 through 10) when they are used as significant statistics or as technical measurements:
Ethel Kassarian, 38, has been promoted to executive director of marketing services in our Houston office.
The attached printout projects the amount of the monthly retirement benefit payable at the age of 65.
A computer literacy program is being offered to all 9-year-olds.
This insurance policy is specially tailored for people in the 50-plus age group.
You will be violating the law if you disregard the job application of a person aged 58. (Not age 58.)

Spell out ages in nontechnical references and in formal writing:
My son is three years old and my daughter is two.
Shirley is in her early forties; her husband is in his mid-sixties.
Have you ever tried keeping a group of five-year-olds happy and under control at the same time?
We've got a surprise party planned for Jack when he reaches the Big Five-Oh.
-Gregg Reference Manual sec. 433-434-