18 or 24 depending if they are attending college and you paying for it or signing the FAFSA forms associated with the student loans. If the child is disabled the child could be a dependent for life. If they are earning money there is a cutoff limit. Check with your tax advisor for the amount for the current year.

If your child is over 24 years of age, has no income, lives with you, and totally dependent on you for their support, then you can claim your child as a Qualifying Relative. This situation is very common for parents who have children who graduated from college but cannot find a job.