If he is a full-time student, up to age 23 as long as he does not pay more then 1/2 of his own support and lives with you except to attend school.
If not a full-time student, then there is no age limit as long as you pay more then 1/2 of his support and his gross income for the tax year is less than $4,000.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**