I don't think that being over 70 is enough in itself to get someone a slot. Sainsbury's are not, as far as I know, taking new customers for delivery, and neither are Ocado.
Sainsbury's allocate slots to the vulnerable (regardless of age) first, then release them to regular online shoppers. If there are any left, they go on 'general release' - ie to people who have not been regular online shoppers, and are not on the vulnerable/shielded list.
I think that this is fair. If you are vulnerable, you are looked after (if the system works!) and should either get something arranged by the council, or a priority supermarket delivery slot, regardless of how you shopped before the virus.
If you are not vulnerable, but have shopped online regularly, you can continue to do so without the slots being overwhelmed by people who usually prefer to shop inshore. From the point of view of the supermarkets, this makes sense, as when this is over they will want to hang on to their regular online customers, who will appreciate the loyalty.
From the point of view of shoppers, if you have used online shopping for years, your reasons for doing so won't have gone away because of the virus. Maybe you live far away from a supermarket. Maybe you don't drive, so can't manage click and collect, or carrying shopping home. Maybe you are agoraphobic. Maybe you are a single parent with young children. Maybe you work long hours and aren't available during the day. Maybe you are a carer. Maybe you just like to shop online.
There are many reasons why people use delivery, and they are by no means all health or age-related.