This pandemic is leading to an unprecedented degree of governmental intrusion into personal liberties that are at the core of constitutional protections, such as the right to gather together and the privacy of one's home (orders limiting the number of people from different households). That isn't to say that the restrictions are unconstitutional, just that we don't know how legal challenges to them are going to shake out.
As long as the hard facts justify government actions and they are limited in time and scope, I think most will survive constitutional challenges. On the one hand, age is a protected class, but on the other, government intrusion into personal liberties must be the least restrictive possible in order to reach a valid government goal. This virus is disproportionately impacting the elderly, so would the least restrictive measure be to isolate them? I don't believe we'll ever face this problem because, as several posters have pointed out, effectively isolating only the elderly is not possible given the way our society is set up. And in any event, people wouldn't stand for it.
That said, it makes sense for people in the most vulnerable age groups to take extra care to isolate as much as possible in order to protect themselves and to get vaccinated as soon as possible.