Davinia I think you make an interesting point (although maybe not for the reason you think).
If somebody has high blood pressure, cholesterol or blood sugar, any medic would be negligent not to tell the patient. If losing weight, exercising more, stopping smoking and cutting down on alcohol would help the conditions, the patient should be told and then has the choice to do something about it. I have read numerous posts on GN and elsewhere suggesting that people who choose not to help themselves are perhaps not deserving of NHS treatment if they subsequently become ill.
I see parallels with the Covid-19 situation. People should be told that their age is a risk factor for being affected badly. Obviously, they can't do anything about their age, but they can do something about reducing their risk of catching it. That's not to do with the "government telling them what to do" but looking after themselves. The NHS will try to treat everybody but, sadly, the survival rate for some groups is not good and resources might need to be prioritised.
As others have said, people are not being told to lock down (shield) merely because they are over 70. They are being advised to do the same as almost everybody else, but they are strongly advised ie. don't cut corners, don't make excuses, don't think that just one hug or meeting a friend won't make a difference, make absolutely sure you don't touch your face when out and about. This is the same advice as people under 70 with underlying conditions are receiving.