*It is also possible that he will spend it on travel
What!! A young man of 18 spending his own money his granny left him to see the world and have fabulous life changing experiences.....that would be an outrage! hmm*
yes well you are more zen than I am. My mother left about 100k in total after a lifetime of saving, paying a mortgage, working. I work and have done since leaving school/college, I pay my mortgage and save - for my kids upbringing, education, and my retirement. I hope to leave something to my children like my mother did that will give them a hand up when they need it.
If my 18 year old (I have one) took the money my mother left after a lifetime of saving/paying mortgage and spent it on travel, while I myself was still (like most people) worrying about my pension and still paying off my mortgage, yes I would think "what a waste". no matter how "life changing" the experiences were. I couldn't do anything about it but that is how I would feel.
There are loads of threads on here debating inheritance tax which talk about the fairness or not of inheriting. The point everyone acknowledges is that inheriting from the previous generation generally gives you a leg up/start that you wouldn't otherwise have. My parents would talk about it ("well maybe what is left will educate the children" etc) - I think it gave them pleasure to think they would help. Personally I am not saving and being financially careful to fund an 18 year old's travel. If my son wanted to do it, I'd be fine but I wouldn't fund it. I am funding his university education though.
Of course in this case the money is the grandchilds-he can do what he wants with it, blow it on the horses if he wants. And maybe his grandmother was more like you and would have approved of whatever he did. I wouldn't.