I've had random failures on retail payments using my phone which is based on a loaded card. I just use another card and the phone usually seems to sort itself out fairly quickly.
I too am old !
I use an Oyster Card, as recommended here, with a Senior Railcard loaded on to it. It definitely saves a lot of cash. You have to get a staff member at a Tube station to load the Railcard onto the Oyster, and that's best done as soon as you get the Railcard. You can load cash onto the Oyster online, but I prefer to do it at a Tube ticket machine, or in fact at any railway ticket machine in London that has a yellow Oyster reader on it. It's just as quick and convenient.