If it helps at all, I went from hairpiece to sly a week and a half ago and most people are surprisingly saying I look younger and thinner! I never would have guessed at that reaction.
Oh, and a lady at work has taken to calling me "sexy head" now!
Definitely all about attitude. I started losing my hair at 19 and had similar concerns about "looking older," but honestly it has never been an issue. Not once. How "young" or "old" you are is largely a result of how you carry yourself. If you are confident, that's all that matters. A smile doesn't hurt either.
Yes, last week a long lost friend commented that I didn't look a day over 65. Smart Arse!
I'm assuming at your age, your beard has no gray or white in it, so a shaved head will make you look older, but not aged. As others will tell you, try the sly look and wear it with confidence. If you decide you don't like it, your hair will grow back fairly quickly. My guess, though, is that once you see yourself sly, you'll stay sly!