"How old do you think I am?"

21. Younger than 21. Or close to 21. Which is why I just asked to see ID. 
And by the date on your ID, I wasn't far off.  You seriously JUST turned 21, so why are you acting so shocked that I had the 'nerve' to card you?  And if you are way over 21, why are you offended?  You should be flattered; I think you are younger than you are.
Rule of Thumb:  No customer is more important than my job.  Sorry.  I am not about to not card you just because you act offended when I ask.  In fact, it is going to make me study that, cheap, fake ID even harder.  In case you had not realized, TABC will not only suspend my license to serve you, I will lose my job, and face serious criminal charges.  So excuse me, but why did you ever think I would take a risk and not card you?

"Can I get a Keystone on Draft?"

No, but thank you for informing me that I will not be getting a tip.  Don't get me wrong, Keystone may be the drink of champions, not this champion, but some people genuinely like it.  That's great, especially for the creators of Keystone.
For me on the other hand, not so great.  Why? Because if you order Keystone, you might as well order water.  You can get any beer on tap-- yet you choose to order the cheapest beer on tap.  It's like going to a top knotch steakhouse and ordering the chop steak. 
Please, at least try to act like you have had better beer in your life and order, well, really ANYTHING else.