I've seen root development on current-season suckers, but usually late enough in the season that the bark is brown and the stem is pretty rigid. However, since one of big benefits of air layering is the comparatively large plant you can end up with (as opposed to rooting cuttings), you might be better off waiting until the target branch is big enough to work to your advantage.
Here are a couple of examples; both are Celeste cuttings Cecil sent to me last year, of the same age. I rooted one directly in the ground, and the other I grafted onto an FMV-laden UCR 135-15s--and am now in the final stages of removing it via air layer. The in-ground cutting is healthy enough, but very small by comparison. I could have air layered the grafted plant several moths ago, but would have lost all the growth generated by giving the mature root system enough time to work its magic.
Unfortunately, I didn't consider the high probability of transferring FMV to the scion when I grafted it, so in this case I'll keep the rooted cutting, even though it's so much smaller than the air layered graft.
