Do old people change shape?

I bought the Senior Cat a new pair of "best" trousers on Saturday. This involved a trip into the city to the only shop likely to have something that would suit him. I hate the sort of shopping which involves trips into the city. This did not prove any different.
It is so long since I have been into that particular department store that I had no idea where to find "menswear" but a sympathetic staff member explained and I prowled up to the second floor and, finally, found the right area. 
Yes, nice wool-polyester machine washable, ultra conservative grey trousers. A helpful staff member who seemed to know what he was talking about took the old pair that needed replacing and found the right size. They were as close as we could get to the old pair  - also wool-polyester machine  washable,  ultra conservative grey trousers.
I bought them.
I then rewarded myself by spending the birthday/Christmas voucher my brother and SIL had given me - on a much needed new summer dressing gown. (The old one was falling to pieces. This is how much I detest clothes shopping.)
I went home.
No, the trousers did not fit. The Senior Cat said, "Well, the others are a bit tight."
How can they be tight? He has lost weight since we  bought them about eighteen years ago. I can remember the day we bought them. It was not long after my mother died. I can remember him standing there in the shop trying them on and the assistant putting in a pin where they would need to be taken up. 
But they were tight. So  yesterday I took them back. I meant to have a quick trip in and out in the morning. It shouldn't have taken too long but the train wasn't running. After waiting and waiting I discovered that one of the drivers had gone through a red light at a station down the track - and gone over a set of points at the same time. I don't imagine he will be at all popular.  The railway man trying to get to his shift told me all this and told me to come back later. 
I went in after lunch instead...annoying. I had other things to do. But I took them back. I held my breath. Did they have the next size? Yes!
I exchanged the trousers and prowled back home. I made the Senior Cat try them on. They fit. 
They fit except of course they are far too long. This morning I will ask one of our neighbours, a professional dressmaker, if we can call on her professional services.
And  these so called "machine washable" trousers will not be going into the washing machine ever. The others didn't. I just wish they had lasted another eighteen years.