hyphen or not?

[uniopp]

Which is correct -
a 4-year-old boy
a 4 year old boy
Are there some other cases when we need hyphens?
Thanks

 

[albertino]

Which is correct -
a 4-year-old boy
a 4 year old boy
Are there some other cases when we need hyphens?
Thanks

(Not a teacher)
It is another pattern to express "He is a boy of 4 years old".
The first is preferable, as "4 years old" is put before the noun as an adjectival phrase.

The second question is a BIG one, you'd better raise it with a new thread.