If you are asking about the $500 stimulus then the child must be under the age if 17 (no older than 16) at the end of 2020.
There are two types of dependents, Qualifying child (up to $2000 refund) and Qualifying relative ($500 refund).
The requirements are very different. Age 18 and full-time student is probably a Qualifying Child.
---Tests To Be a Qualifying Child---
(Must pass ALL of these tests)
NOTE: If a child passes all of these tests he must say “yes” on his/her own tax return (if he/she files one) that another taxpayer CAN claim him/her as a dependent even if they DO NOT claim him/her)
1. The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother,stepsister, or a descendant of any of them.
2. The child must be (a) under age 19 at the end of 2019, (b) under age 24 at the end of 2019 and a full-time student* for any part of 5 months of 2019, or (c) any age if permanently and totally disabled and must be younger than you (or your spouse if filing jointly).
3. The child must have lived with you for more than half of the year (There are exceptions for temporary absences such as school, illness, business, vacation, military service).
4. The child must not have provided more than half of his or her own support for the year.
See Worksheet 3-1. Worksheet for Determining Support
https://www.irs.gov/publications/p17#en_US_2019_publink1000171012
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child is not filing a joint return.
7. The child must be a U.S. citizen, U.S. resident alien, U.S. national, or a resident of Canada or Mexico
*A full-time student is a student who is enrolled for the number of hours or courses the school considers to be full-time attendance during some part of each of any 5 calendar months of the year.
See IRS Publication 17 for more information.
https://www.irs.gov/publications/p17
*****************
---Tests to be a Qualifying Relative (& Unrelated Persons)---
(Must meet ALL of these tests to be a dependent)
1. The person cannot be your qualifying child or the qualifying child of any other taxpayer.
2. The person either must be related to you, or must live with you all year (all 365 days - There are exceptions for temporary absences such as school, illness, business, vacation, military service) as a member of your household.
3. The person's gross income for the year must be less than $4,200 (tax-exempt income, such as certain social security benefits, is not included in gross income)
4. You must provide more than half of the person's total support** for the year.
5. The person is not filing a joint return.
In any case, the person must be a U.S. citizen, U.S. resident alien, U.S. national, or a resident of Canada or Mexico
The above is simplified; see IRS Publication 501 for full information.
https://www.irs.gov/publications/p501#en_US_2018_publink1000220939
** Worksheet for determining support
https://www.irs.gov/publications/p501#en_US_2019_publink1000226268
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
