can't really blame the children if their parents never taught them to always send a 'thank-you'...
my neices (in their 20's now) have ALWAYS sent a brief message-some sort of note to let me know they received a gift- regardless of circumstances- they were taught as very young girls to sit down & write thank you's -- my own kids (now in their 30's- and their children---teens) always sit down & write notes of thanks-- during the quiet time after birthday parties- in the afternoon on Christmas---when ever they have a few quiet moments-
i still always set down & write a note letting people know i am thinking of them---it's all in the way a person is raised.
as for 'age to quite giving'.... i don't really think an age should have much to do with it- gifts are supposed to come from the heart- not be a commitment-unless your family draws names or has some (tradition) way to handle the adults-
in our family when the kids started having kids- the (kids) always receive a gift from anyone who can afford-wants to give one- no one is expected-forced to give gifts-
as for the adults- sometimes some of us give gifts to adults in the family- again- they are not expected- no one feels slighted if they do not get one- and someone else does- we have a large family- sometimes we get lots done & many people get gifts, sometimes time/finances restrict that- everyone works, has lives- know how things are- its a gift- not an expectation- and everyone says thank you- all that being said- my youngest son (in the navy- busy) is the worst about the thank-you's...he just seems to have other things on his mind- it does not mean we stop loving him,,or refuse to give to him...when we have sent him something & weeks go by without hearing from him we generally give him a call- & ask---did your package arrive ok---then he always says---yes- i loved it! thank you!...
giving should come from the heart- if your heart is not in it-it's meaningless- so don't bother-
we were taught to not give with expectations attached; just like giving to charity---give because the act of giving is reward enough---not the 'pat on the back from others'