If we claim our daughter who was 18 at the end of 2020, will she be eligible for any retroactive economic stimulus payment?

No, your daughter won't be able to get the Recover Rebate Credit on her 2020 tax return if she can be claimed by someone else as a dependent. If she meets the requirements to be claimed by someone then she won't qualify for the Recovery Rebate Credit, even if the person decides to not claim her.

Yes, you should claim your daughter if she qualifies to be your dependent.

Please review the requirements for your daughter to be your dependent:

Qualifying child

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