This could be challenged on 2 grounds. Firstly, age discrimination legislation, particularly an arbitrary age limit which would be difficult to police. Secondly, there is little logic to it as younger people are much more likely to be carriers of the virus, as they are much more mobile in terms of work and socially, both home and abroad. And those elderly people who feel at risk are probably already taking precautions.
I could vaguely understand a complete behind closed doors approach, but a partial one is a nonsense.
And on the same basis they should also close down the House of Lords, whose average age is well over 70!