The time to apply a systemic insecticide is pretty much now. You need to examine the undersides of the new leaves regularly, and watch for the first signs of nymphs or adults. They may already be active, so don't wait. If you start treating after you see stippling damage on the tops of the leaves, it is too late. The damage is done for this year and the populations are already high.
Yes, your older Rhodies may be quite susceptible to them. If you love Rhodies but want less lacebug problems, consider species and cultivars with a fuzzy indumentum on the undersides of the leaves. This is not only a highly ornamental feature, but strongly discourages sucking insects. Some lovely examples are Rhododendron pachysanthum and R. 'Golfer'.
For more information see the SolvePestProblems website, https://pestd8.prod.acquia-sites.com/plant-problems/azalea-rhododendron/lace-bugs