There is no age requirement for a dependent to be eligible for the 3rd stimulus payment.
There is an age requirement for the Child Tax Credit. The dependent must be under the age of 18 in tax year 2021. If they are age 18 or older they are eligible for the Other Dependent credit of $500
IRS website for third stimulus payment FAQ's - https://www.irs.gov/newsroom/questions-and-answers-about-the-third-round-economic-impact-payment-top...
You were not eligible for a payment if any of the following apply:
- You could be claimed as a dependent on another taxpayer’s 2020 tax return
- You were a nonresident alien.
- You did not have a valid SSN issued to you by the due date of your tax return (including an extension to October 15 if you requested it).
