When do you get "old"?

This is a question which has been occupying my thoughts on and off for the last few days.
The Senior Cat will be 95 on Monday. I've noticed a real difference in his physical capacities over the last twelve months. It has reached a point where, if he is home alone, I try not to be away from the house too long. I do the shopping and other necessary chores while someone helps him have a shower. I go to the library while he is having his afternoon "rest" - and now he sleeps in that time.  He goes to bed earlier - and gets up later.
None of that matters of course. What does matter is that he can no longer get down to tie up his tomato plants or use the circular saw in the shed or, more serious still, ride his gopher to get to his church or the hardware shop. I do the banking.
In that sense I suppose he is "old". On the other hand the Senior Cat is still reading psychology, philosophy, religion, and more . He reads about social issues - and discusses them with me. He taught the two little kittens across the road a conjuring trick recently - and may well teach them some more. He was discussing 3D printing with a young adult recently. Those things still fascinate him.
In other words he still has all his mental faculties - even if I have to repeat things because he didn't hear something the first time.
But, he laughed at himself yesterday. 
He had been trying to work out how to provide his grandson with the wedding gift he has been planning and when it would need to happen.
"I'll have to get you to put some money in the cheque account," he had told me.
I looked at him and shook my head. His grandson has never used a cheque in his life. He has probably never even seen one. 
"No," I told him, "You just put it straight into their joint bank account."
He stood there thinking about it and then said, "I'm really out of date aren't I?"
He isn't really. For someone of almost 95 he is still "young" in many ways. He can use an i-pad and his new mobile phone is a thing of joy to him. It is just that things like 21st century ways of doing banking are a mystery to him.