One of the most annoying formulations anybody ever offers in an argument is to imperiously interject, "Who are you to say [such-and-such]?"
There is, of course, no logical rebuttal to such a nonsensical question, which implies that you not worthy of even engaging in a discussion of the matter at hand.
There are, however, several things you can say which point out the absurdity of such a line:
"What would you like me to do, whip out my resume and recite my qualifications before offering an opinion? I could ask the same of you, but don't you think that would be sort of ridiculous?"
Or, "Who am I? I'm someone who's observed [such-and-such] his entire life."
Or, "The subject of discussion isn't me, as much as you'd like to turn this debate into an ad hominem attack. It's [such-and-such]."
Or, you can mock them by thundering back, in even more imperious tones than they used, "Who am I? Who are you to ask such an impertinent question?!"
Of course, people who'd use such a moronic line in the first place probably aren't even smart enough to see they're being made fun of.