I'm 45. The past year my mileage has been quite light, maxing out around 55-60 miles, likely averaging more like 40-45. So while I have run some second runs (maybe once a week on average) I wouldn't say I am typically "running doubles" these days.
That said, until last fall, the previous 4-5 years I average more like 65 mpw, generally with a handful of 100+ weeks per year. I typically ran 8-12 times per week during that period. That I would characterize as "running doubles" as a master for a few years.
I hope to ease my mileage back up over the next few months, and this will include some second daily runs.