Age is a factor when claiming children as dependents on tax returns, but the Internal Revenue Service indicates that a child or other relative may qualify as a dependent at any age if they meet certain conditions. The agency lists those conditions in IRS Publication 501. Among other things, children may qualify as dependents if they are full-time students or if they depend on their parents for most of their financial support.
You can claim a child as a dependent if he was younger than 19 at the end of the year for which you're filing taxes. You can claim him until age 24 if he’s a full-time student during the tax year. In either case, the IRS requires that a child be younger than the person claiming him and younger than that person's spouse in cases where a married couple files a joint tax return. The IRS allows a taxpayer to claim a child at any age if the child is completely disabled on a permanent basis.
A child must meet other conditions to be eligible as a dependent, outside of age requirements. For example, you must provide more than half of your son's total financial support to claim him as a dependent. Your son also must be a U.S. citizen, U.S. resident alien, U.S. national, or a resident of Canada or Mexico for you to claim him as a dependent. Furthermore, just one taxpayer can claim the same child as a dependent. The IRS may disallow your claim if someone else lists your son as a dependent on a tax return.
You can't claim your son as a dependent if he files a joint return with a spouse, unless the return is filed only to claim a refund and neither spouse owes taxes on other returns. For example, the IRS indicates that a taxpayer's 18-year-old son and his wife may have just $800 of interest income for the tax year, but taxes were taken out of their interest income. In such cases, they could file a joint return just to get a refund for the taxes that were withheld. In this case, a parent would be able to claim the son as a dependent if all other qualifying conditions are met.
You may still be able to claim your son as a dependent under the IRS conditions for qualifying relatives if he doesn't meet all conditions as a qualifying child. That's especially true if you provided more than half of his financial support for the tax year. However, you also must meet certain IRS requirements to claim dependents on your tax return. For instance, you're ineligible to claim a dependent if you can be claimed as a dependent on another taxpayer's return. You also can't claim your son as a dependent if you file a joint return with your spouse and your spouse can be claimed as a dependent by someone else.