Interesting topic.
My own experience is that I don't expect gifts from guests and the majority don't give me anything, but there are still a reasonably large number who do and this doesn't seem to have anything to do with their age.
However, I have noticed that Asian guests (particularly Chinese and Korean) are the most likely to bring gifts - some European guests do but Americans pretty much never do. I think that in many Asian cultures it is standard that you would bring a gift when visiting someone's home (I know that is the case in Pakistan, where I am originally from). It is not necessarily a sign of appreciation, as they have usually bought the gift prior to staying (often something 'typical' from their country), but is just considered good manners. The gifts have ranged from something small, like a box of tea, to expensive, like a very beautiful tea set.
Guests will more often leave thank you cards with a personalised message at the end of the stay if they want to show their appreciation. Most often, they choose cards with pictures of cats on them (I have three kitties, who are very popular with guests).
I haven't noticed a decline in gift/card giving, other than I have hosted less Asian guests lately than usual, and more Americans and Europeans (maybe since I switched to Instant Book?). Of these, only the Europeans have left gifts. Recently, some Italian girls bought me flowers, some Italian guys a boxed set of wine and a little book from Sardinina and an English mother and daughter very thoughtfully gave me a pretty guest book.
Before that, I used to display the thank you cards on a sideboard in the dining room, but I don't know if guests realised what they were or peeked inside of them and my cat was constantly knocking them over! Now, I have stuck the cards inside the guest book, which I leave with a pen on a table by the front door. I am finding that guests are leaving thank you messages in the book without being prompted, which is lovely.
I agree that behaving respectfully during their stay and leaving a good review are more important that giving the host a gift (and not all guests that have given gifts have done this). However, I do always appreciate it when I receive a gift or thank you card. I see it as a bonus, especially if you can see some thought went into it.
In conclusion, I don't think gift giving is declining, but it is sporadic. Asian guests most commonly bring gifts, followed by Europeans. Age seems to have nothing to do with it.
