Deductions & credits

I doubt that this comes up very often because it is extremely rare for a 17 year old to provide more then half of their own total support for the year with their own money.  ( I suppose can happen if the child received an inheritance or other money - but it is rare.)

Support includes food, housing (fair market rental value divided by number of occupants)  and other living expenses.. 

See Worksheet 3-1. Worksheet for Determining Support
https://www.irs.gov/publications/p17#en_US_2020_publink1000171012

If the child meets the requirement of a dependent then the child CANNOT claim them self even if not claimed.  Tax law does not allow that.  That is why there are two question  in the interview "CAN you be a dependent of another taxpayer?" and "WERE you claimed by that tax payer?"  Both must be no to claim themself.

---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 2020, (b) under age 24 at the end of 2020 and a full-time student* for any part of 5 months of 2020, 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_2020_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