Rather than lockdown the over 50s/ over 60s / over whatever they decide, would it work if say pubs/cafes only allowed over 50s/60s/whatever on Sundays/Mondays - the older age groups tend to follow the rules and they could make it super safe. They would be able to meet friends/siblings so not completely isolated, although obviously, the compromise is not mixing the generations.
That leaves 5 days a week for the youngsters to socialise.
Seems fair.