Honestly?

Please stop being a tone def clod.

“Well hello there young lady!” addressed to anyone over six years old, yes, it's patronizing. Lady is used to put women in a category of disenfranchised, powerless, considered proper as long as she doesn't poke her nose or opinions into the affairs of real men. At the best of times, it's overly formal, old fashioned, and a little bit goofy. At worst, it's snobbish and insulting. It says, I’m putting you on a pedestal because you're too fragile and not quite bright enough to deal with the rough and tumble world. Or it says, know your place, oh fragile one, beneath my strong embrace you will be safe. I’ll protect you, you can’t do it for yourself.

By the time a girl is a woman, there are any number of more respectful ways to speak to her. If you can't think of anything better, Ms. is a general purpose, all ‘round form of address, unless the person in question tells you they prefer something else.

Unless, of course, the person you are addressing actually is a lady, because she holds a title where the form is appropriate.

If you're planning to write a comment about respect and chivalry, keep your romantic Victorian notions to yourself. Good manners are timeless, and apply to all genders.