Can I can I choose not to list children over 20 as dependents even if they meet all 6 qualifications as dependents.

you don't have to claim any dependents.

While the answer "no" you don't have to is correct, it's not complete.  The question shouldn't be do you have to, but do you want to.  The ideal scenario would be to determine the best scenario for the family as a whole.  I.E. what is the total tax paid if you claim all of your legitimate dependents as opposed to your filing without and the dependents filings for themselves.  If the family is better off with you claiming the dependents and the dependents are willing to forgo their own filings...I would think they can file an amended claim to change their status and allow you to claim them if they are indeed legitimate dependents on your claim.

@gsbowers49 

Your thinking is the way many people approach the problem.

However, dependents are not allowed to state they cannot be claimed, if that is not true.

The rule is often ignored.