Adult only (21+) wedding

I wouldn't put that on the invitation. Just address the invitation specifically to the people who are invited (Mr. John & Mrs. Jane Doe.... NOT "The Doe Family") to avoid confusion.

Then on your website you can say something like "our wedding will be limited to adults ages 21+"

If people give you a hard time about it, you can maybe tell a little white lie and say it's a venue liability since there will be alcohol provided.

I would just address the invitation to people 21+ and on the RSVP card put "____ out of _____ will be attending" and fill out the second line for the exact # per household invited. You could also put it in FAQs.

I would maybe add something on the website that it's 21+ and just address the invite to those invited. My cousin is doing this and added a note to their website, but in all fairness her venue doesn't allow anyone under 21 because of the nature of the venue so it was a good excuse for her.

I would just have it say "Adults 21+ only affair" on the bottom of your invitation.

I don't think I would put it on the invitation itself. I would think that a note on the RSVP card would be sufficient.