Ah but weather is not re!iable. You do not say where you are located but, what you describe can happen if winter is too cold or if winter is mild enough to break dormancy... just as temps crash well below freezing. I had that last problem this year: my hydrangeas in protected locations managed to bloom but many in a northern exposure look like yours. :o(

A 20 y/o hydrangea should not be considered old. 75+ would be old.

If the shrub have any sentimental value, June would be a good time to get cuttings and create new plants.

There's a bunch of reasons why performance can suffer.

Cut out as much dead wood as possible as well as any weaker looking portions or permanently wilted foliage growing from old wood. And hit the plants heavy at least once a growing season with 10-10-10, high phosphorus stuff 10-x-10 and/or spent coffee grounds. I make sure mine get a good dose of water at least twice a week. It rained about 6-10" in May here (which required little if any supplemental watering) and while doing some recent transplanting the root balls were fairly dry. So aside from areas with spongy ground or standing water, there's really no way to over-water a hydrangea. It really makes these plants take off! Overall, the soil should also be very dark and rich, heavily worked and somewhat amended to at least a depth of a foot (no huge chunks of clay or tremendous amounts of root competition from trees, etc).

Another tidbit on late Spring freezes - they can zap buds that *were* going to become flowers, leaving other ones to just grow foliage alone, even on old wood. This happened with about 30 of my macs this Spring, including an April night of 19F (and 27F once my containers were back outside).