I have a 12 YO DD, and she writes the best thank-you notes. When DH's family was over for Christmas, she sat down and wrote them right after the last guest left. These were in the mail on 12/26!
It wasn't always this easy, just the last couple of years. She really makes them personal too. I.e. this year she received a small GC to a cool candy store that DD really likes. She mentioned to the giver that she will be keeping a close eye on this GC, just in case her dad felt the need to "um, take it off her hands." Now, DH would NEVER do that, but DH is also known for having a sweet tooth on occasion.