That pretty much AIBU that they didn't say thanks?
My niece + nephew both turned 18 in August. I posed them each a card with €40 but I never heard anything back. I was then down home at party over the weekend and met them. I was chatting to them and asked how their birthdays went, they celebrated together, and said 'did my cards arrive', niece 'ya it did', nephew just nodded. I'm a bit pissed off that they didn't say it did, thank you. It's a while since I was 18 but I am fairly sure I said thanks when someone gave me a card with money!