The most effective comeback to an insult is silence.
I'd love to ask how old you are, but unfortunately I know you can't count that high.
I don't know what makes you so dumb but it really works.
Hi there, I'm a human being! What are you?
Shouldn't you have a license for being that ugly?
If you can't ignore an insult, top it; if you can't top it, laugh it off; and if you can't laugh it off, it's probably deserved.
Anybody who told you to be yourself simply couldn't have given you worse advice...
Brains aren't everything. In fact in your case they're nothing.
The only graceful way to accept an insult is to ignore it; if you can't ignore it, top it. If you can't top it, laugh at it. If you can't laugh at it, it's probably deserved.