Professor being mistaken for a grad student
I would take it as a compliment to be mistaken for being younger. And, even if you don't actually feel complimented, that's a good way to respond. For example, you could laugh and say "Thanks. I wish I was still a young and carefree student, but, unfortunately, I'm a professor." 

If you make it appear like you take it as a compliment, people won't feel like they have offended you and it is less awkward. The other party may also intentionally be erring on the side of underestimating your age, because it's seen as less of a faux pas that way.

(I often get people commenting on my height. I always act as if I take it as a compliment and that seems to work well. However, in reality, I'm pretty bored of discussing my disinterest in basketball.)

At conferences, you always have the option of putting "Prof." on your nametag. There is a small risk that this looks pretentious relative to the norms of your community, so it's a tradeoff. 

Finally, there is one sure-fire way to appear older.