The thank you sign is a great gentle introduction to manners. It is great to get older babies in the habit of saying "thank you" when given something. The thank you sign is also a great way to remind older kids to be polite when out in public without being overbearing or embarrassing them.
This is a wonderful sign to introduce early in life as we begin to teach our babies the meaning of gratitude. This sign is best learned by example and consistent use.