I am not clear regarding how much of his investment portfolio he decided to put into bitcoin, and at his age, I am not even sure if he can confidentally assert that he has a 5-year investment horizon.  So I would have a hard time recommending even my common asserted range of 1% to 10%, because with such a likely short investment horizon, he should not be considered as really investing, but instead gambling on the shorter term, and of course, he could end up being correct... but I don't really agree with gambling, especially for 83 year olds... so just given the fact that his investment time horizon is likely only 2 years, I  would recommend 1% to 3% for the average 83 year old, and maybe only a higher portion if he really could afford to lose the whole amount invested because he is otherwise way more wealthy than he can even expect to spend, then if that is the case, then maybe he could go above the 3% range - and surely, he would be in a better position to understand his situation than me, but I am just responding to the general idea of an 83 year old and based on that picture he does not necessarily look like he has any kind of genetic gift or perhaps luck that would allow him to have an investment timeline that is greater than a couple of years, at best.