Seating for teens/young adults?

I don't know if I'd go as high as people in their twenties, but I think it's a great idea to put the teens together. Children of guests often don't have opportunities to mingle so throwing together a party table for the minors seems really thoughtful.

It depends. If there are enough in this age range, teens are often seated together. If not, they are can be assigned to tables with family or friends. Young adults are usually with friends or family members.