Yes - you can still claim him as your dependent if you meet qualifying child rules or qualifying relative rules; there are no age requirements if the child is considered permanently disabled for tax purposes.
Per IRS: "You are permanently and totally disabled if you cannot engage in any substantial gainful activity because of your physical or mental condition. A qualified physician must certify that the condition has lasted or can be expected to last continuously for 12 months or more, or that the condition can be expected to result in death".
To be qualifying child:
- he must not file a joint return with another person
- he must be your son, daughter, stepchild, foster child, brother, sister, half-permanently brother/sister, stepbrother or a descendant of any of them
- he must be under 19, or under 24 and full time student OR permamently disabled
- he must not provide more than half of his own support
- he must live with you for more than half of the year
To be a qualified relative:
- he must not earn more than $4,050 for the year (taxable income, SS benefits do not count)
- he must not file a joint return with another person,
- he must live with you entire year (if not related)
- you must provide more than half of his total support
