My two are 18 and 21 and still do them, mainly to relatives/family friends who we don’t see much - the ones they did this week were very brief, but they did them. I have an excess of Thank you cards so probably when they are used up they will move to doing texts etc more. We had a lovely one from a relative of DH (early 20s) this week, it is nice to receive too.
DS did complain about thank yous a couple of years ago, I pointed out thats fine, I’d have his presents/money and do them instead! He did them.
Some of the people we buy for (mainly teens/young adults) don’t acknowledge at all, not even a text or email, which is annoying when a present has been posted. Often I don’t know if a birthday cheque has been received until it is cashed.
We bought birth presents for two new babies in family in 2018, recieving lovely thank you notes from the parents. Nice to get the acknowledgement and know the present arrived.
My DDs godmother (my old uni friend so not ancient) has commented many times that she loves that my DC send thank yous, in stark reality to her nieces/nephews and their kids who never acknowledge presents. I get Xmas presents from her and 2 other friends, and always thank them as part of a text on Xmas or Boxing Day. Basic manners.