Very sorry for your loss, but glad you are there for your father.
As far as being too old I think it's more a matter of his overall health and physical condition and if there is any reason for concern check with his doctor first before trying a new activity like metal detecting.
Having said all that I'm pretty sure there are likely some members on the forum that might be in their 80's (maybe even older) and still metal detect, some of them might have to be more careful during hot weather and also might not hunt as many hours at a time as when they were younger, but again, it needs to be based individually on each person's individual health and overall physical condition.
EDIT TO ADD: my response to your concern was based on the impression I got that your dad wants to consider metal detecting, but if I am wrong in that assumption then it does need to be something he really wants to do, and when he is ready to try. (but even if he wants to, and you have a legitimate concern for his health, it simply seems wise to check with his doctor to be on the safe side, but please realize I am not trying to tell you what to do, but merely offering a response based on the concern you expressed)