"How can I make old people realise that they are deaf?"

This was how the question was put, but it could better have been put as how to help someone, anyone, realise that they are deaf. It is an important question because once people understand that a problem exists, they are far more likely to solve it themselves or be agreeable and grateful when others try to help.

Essentially what you must never do is to keep on telling someone that they are deaf. They will only think that they are being got at, particularly if the deafness has come on gradually and they just think that everyone else is mumbling. 

When faced with the question, I asked myself how I became aware that I was deaf. In fact it was patently obvious and still is. With a group of people scattered around, as in a restaurant or lecture theatre, I am perpetually reminded when one person speaks to another just as far away from me and on the other side: I often think, "They'll never hear from that distance", and - lo and behold - they answer as if there was no problem at all.

So the strategy I would suggest is to enable deaf people to come to their own realisations that they are deaf, by setting up situations which show that other people are hearing and they are not. This is impossible for a single individual to do alone, so invite others along with the deaf person, arrange for the seating to be not too close together, seat the deaf person in the middle and let things take their natural course. If the deaf person joins in with the wider group, perhaps they aren't as deaf as you thought. If they don't join in, they won't need very many such experiences before the realisation dawns and they accept their deafness and that something must be done about it.