Addressing invitations

We did "The Smith Family" if the kids lived in the same house, regardless of age. If they were older and lived at home, they still all got the 1 invite.

Very few people use inner envelopes anymore so you are not weird for not doing so. I didn't either. For families with children, and you are inviting the entire family, it is fine to say The Smith Family. Alternatively, you would list the parents' names on the first line and then immediately below on the second line list each child's name "Jane Smith" and "John Smith."

For your second situation, etiquette would have you send separate invitations to any member of the family who is 18+ even if they are addressed to the same place. This is so it will be very clear if those individuals can bring a guest, or if that person is dating someone then it would be addressed to that family member and their SO by name.

I'm doing the same. I have a few older cousins who have kids older than me and I'm not sending them their own invitation. I'm also not giving them a plus one so it seems like a waste to be like here is an invitation for one to the same house as your parents and little siblings who also got an invitation.

I still get family invitations to my parent's house for weddings and I moved out 7 years ago.

Same! I have lots of cousins who are over 18 but live at home during school breaks. I didn't receive any confusion when I would send it to "The Jones Family" and that included the adult children.

We didn't do the whole sending separate STDs to over 18 living in the same house as their parent and won't for invites either. Not a fan of wasting paper, because that's what it is, a waste of paper and waste of money. Chances of the 19 year old reading the mail is low if their parents already have one too. Yes, we are using "The X Family" on the front envelope because I haven't really seen people use two envelopes anymore!