To quailfy as a surviving spouse in 2022 does my dependent child have to be under 26 years old?

No.  The child's age is not a factor. Below are the eligibility rules extracted from Publication 501 for filing status Qualifying Surviving Spouse. 

You are eligible to file your 2023 return as a qualifying surviving spouse if you meet all the following tests.

1. You were entitled to file a joint return with your spouse for the year your spouse died. It doesn't matter whether you actually filed a joint return.
2. Your spouse died in 2021 or 2022 and you didn't remarry before the end of 2023.
3. You have a child or stepchild (not a foster child) whom you can claim as a dependent or could claim as a dependent
4. This child lived in your home all year, except for temporary absences. 
5. You paid more than half the cost of keeping up a home for the year. 

If the child isn’t claimed as your dependent in the Dependents section on Form 1040 or 1040-SR, enter the child’s name in the entry space at the bottom of the Filing Status section.