What's the right thing to do here? Just go on a nice casual date and then gently tell her I think the age thing is too much?

No. That's not a nice casual date. That's leading her on and then ruining her evening. Or possibly her whole week, depending on when the date is and how excited she is about it.

Sorry to be harsh, but that isn't right. That's not what nice people do. A nice casual date is, a girl gets all excited and dresses up, and she isn't guaranteed to get shot down by the end of the night.

Don't do that. It's not OK. If you're going to be disappointing her one way or the other, the least you could do is show some respect for her time.

As for the bigger question, I'm going to go with the obvious solution. Just be honest with her and tell her "I didn't know you were so young. I got a lot of baggage." If you're going to bail on her, that's all she needs to hear, dude. That's two sentences.

And if you're not going to bail on her, then it's obviously because you decided something purely physical would be fine. If that's where you're coming from, she absolutely has to know up front.

Basically, I think the answer for every one of your questions here is "take responsibility and tell the truth."