You can claim someone older than 18 as a dependent if you meet the requirement of the law.
If the individual is your child, you can claim them if they are a full-time college student and they do not provide more than half of their own support.
If the individual is not your child (or if they are your child and they aren’t a college student) the following must apply: (From IRS Publication 501:(
“They do not qualify as a qualifying child dependent (if the 18 year-old is a college, this may be an issue for anyone other than a parent).
They are either related to you (in one the following relationships) or they live with you the entire year.
Your child, stepchild, foster child, or a descendant of any of them (for example, your grandchild). (A legally adopted child is considered your child.)
Your brother, sister, half brother, half sister, stepbrother, or stepsister.
Your father, mother, grandparent, or other direct ancestor, but not foster parent.
Your stepfather or stepmother.
A son or daughter of your brother or sister.
A son or daughter of your half brother or half sister.
A brother or sister of your father or mother.
Your son-in-law, daughter-in-law, father-in-law, mother-in-law, brother-in-law, or sister-in-law.
Any of these relationships that were established by marriage aren't ended by death or divorce.
They made less than 4,050 dollars for the year.
You provided more than half of their support.”
