How do I buy tickets for local transport in Prague? Particularly for the elderly.
I am planning to self-answer as I expect that this could be useful.
You can now pay by contactless cards (Visa/Mastercard) as well on (almost) all buses and trams. For buses/trams the terminal is in the middle of the vehicle, for metro stations it's located before you enter the escalators. If a terminal is available you don't have to buy a ticket in advance.
You may buy physical tickets for local transport in a number of places such as the main railway station. These are available for a range of periods from short, e.g. 90 minutes (maybe even 30, I forget), up to 72 hours. The longer tickets are good value.
The tickets are valid on the trams and underground (subway) within the city.
You must stamp the ticket on first use. There are yellow machines to do this. The machines are on the trams and at the entrance gates of the underground. For further trips within the validity, just get on. I don't know whether you need to complete your journey within the validity period or just begin it.
Worth noting is that for ages 60 to 64, it is half price; 65 and over is free. Bring some official ID to prove your age in case of inspection. A passport or EU ID card is suitable. No ticket needed for the over 65, just carry suitable ID.
A minor oddity is that there is no physical 72h ticket for 60+. You may ask for it but you will get three 24h tickets with a discount. A little less convenient but potentially better value as you might not need to immediately validate another ticket when one expires.
Some services, e.g. the funicular railway may only be used with a 24h or longer tickets. You can buy a specific ticket but it is about the same as 24h.
You can also pay by app but I have not tried that. For a single trip, it is maybe more convenient.