From what I've been able to gather is it better to be given to the younger crowed because Ive seen anywhere from 57% to 78% effective rates and it tends to go down with the older patients. That's why they're not recommending it for older people
In fact in real situations where it has been used, ie the UK for example, it has not really been less effective for older people. The issue is that in the trials only about 10% of the volunteers were over 65 so the data set is not robust. They just don't know for sure how well it works based on the trials.
Out here one plan is to give the AZ vaccine to younger police and fire fighters. This makes sense because they fit in a relatvely low risk group as far as outcomes are concerned but a higher risk of infection with relatively minor or not symptoms. This is relevent becase preliminary studies suggest that the vaccine may significantly reduce asymtomatic spread.
The other thing about teh AZ numbers is that they are based on parts of the trials taking place in the UK, South Africa and Brazil so they include more exposure to the variants than the mRNA viruses. So comparing raw efficacy numbers is a bit apples to oranges.