Running is harder, much harder, that is why the bicycle was invented.
I very rarely, almost never, see runners that are older than me.(~60), I would say 40-50% of the cyclist that I see on the roads and bike paths are older than me, this might be specific to the area and hot and humid weather (Palm Beach and Martin County, Florida) but this is my unscientific observation. If you are 60+yo even relatively slow 5k running (10-11min/mile) in hot and humid weather is a challenge to your cardiovascular and skeletomuscular systems. OTH, biking has advantage of extra cooling, and easy choice of speed from easy almost no effort ~10mph to somewhat challenging ~18-20mph. The best about running, cycling and swimming is the advantage of cross-training.