I've known 1-2 people (I definitely know one that I can think of off the top of my head and I feel like there's another) who have named sons Shepherd. The one I can think of is a tow-headed 5(ish) year old now, so that's how I think of someone named Shepherd. I can picture it on someone older (I picture a sort of fratty 20 year old), but it tends more toward the younger end of the spectrum in my vision of a person with this name.