I've always been young for my position. I've typically had more trouble with attitude from people who are close to my own age but several steps down the ladder than people who are older than I am.
My technique for older subordinates is to make it clear that that I respect them and earn their respect for me personally (not just my position) before I try to tell them anything. Even then, I try to ask rather than tell whenever possible. It's amazing how well that works. It's sort of like being a green lieutenant in the army, 99% of what you need to know is "carry on sergeant."
I used to do exactly the same thing with younger subordinates, which may have been part of my problem. In retrospect I think my deference may have been misinterpreted. However, I'm getting old enough and far enough up in the hierarchy now that I often expect the younger people to just do what I tell them out of respect for my position (or fear for their jobs). They'll figure out that I didn't get where I am based solely on my looks soon enough.
I can also say that firing people isn't as much fun as you'd think. That said, I had one that I thoroughly enjoyed having walked out the door