Sure, all ages of trees can get greening. Its especially harmful to very young trees. As the trees get older they gain resistance and then resistance declines again in very old trees. I think the recommendation you read was for commercial groves. At some point the benefit from the covers is outweighed by the cost of new larger covers and of taking them on and off to harvest fruit etc. 3 years old is the youngest trees would typically be harvested, by the way so I think that is where that number came in. It should not be an issue for a homeowner who would like to keep their tree alive and is willing to cover and uncover the tree dutring maintenance. Coincidentally, I have been told that the covers do not need to be tied to the tree trunk as was originally thought. A lot of grove owners are leaving the bag open at the bottom end. Apparently, psylids are "lazy bugs" that won't typically fly down then back up. Having the bag lose expedites maintenance and reduces pest pressure from mealy bugs.
