You can claim anyone of any age as long as they qualify :

There are two rules to claim someone as a dependent on a tax return, either the Qualifying Child rules or the Qualifying Relative rules. 

Who can I claim as my dependent?

You can claim a child, relative, friend, or fiancé (etc.) as a dependent on your 2019 taxes as long as they meet the following requirements:

Qualifying child

• They are related to you.
• They aren't claimed as a dependent by someone else.
• They are a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They are under the age of 19 (or 24 for full-time students).
  • No age limit for permanently and totally disabled children.
• They live with you for more than half the year (exceptions apply).
• They didn't provide more than half of their own support for the year.

Qualifying relative

• They don't have to be related to you (despite the name).
• They aren't claimed as a dependent by someone else.
• They are a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They lived with you the entire year.
• They made less than $4,400 in 2022.
• You provided more than half of their financial support.