The rule says $1200 single / $2400 married filing jointly / $500 for dependents under age 17. But what about dependents that are over 17? Will they get the $1200 on their own? I heard a rumor somewhere that they won't be getting anything.
Dependents over 17 are essentially left out. Their parents won't get $500 for them, and they won't qualify for the $1200 themselves.
One option is to evaluate whether they do not qualify as a dependent in 2020. In that scenario they could file themselves and take the $1200 tax credit on their return a year from now.
So, my mother is SS dependent (her only source of income) and normally doesn't file taxes b/c her income is too low. We normally claim her as dependent. My understanding is that in order for her to receive a stimulus check 1) I have to file 2019 taxes NOT claiming her as dependent (which luckily have not yet filed); and 2) she then has to file 2019 taxes, which she would normally not do.
I suspect that dependent adult children would be similar.
Note that technically the person must fail one of the dependency tests (for each category of qualifying child and qualifying relative). If they qualify as your dependent and you don't claim them, they still technically don't qualify for the credit, although that would be something that would probably only come up in an audit.
In early reporting, the indication was that as long as an SS recipient received an SSA-1099 for 2019 (or even 2018), they would get the stimulus payment even if they didn't file a tax return. This seems to match up with the bill's intent. However, I have seen reporting today indicating that a return does now need to be filed; I'm not sure whether to believe that or not. I'd look to irs.gov for proper guidance.
