The thesaurus is failing me in this case. Many of the characters insult this other character with "little girl" and "child" or go the racist route and refer to her as "human" or "mortal." Another character is observant enough to know that she's at least in her high 20's. I can't think of what is as bad as "little girl" but would make more sense.
Another way people often are condescending to young women is the "Sweetie", "Isn't you a cute one?" and so on. Comments that reduce her identity to her cooks or she fulfilling some traditional female stereotype. These might not be condescending calling your girlfriend, but definitively is condescending when a stranger calls you that, or in a professional or other serious situation where you should be respected.
I was wondering about him saying "My dear" in the same breath as threatening to do awful things to her. It was coming off as a bit too much of something. Ah well, just launching into the threats is pretty good.
Leaning forward, resting his thick, beefy hands on his knees in an exaggerated pose intended to resemble a father talking down to a small child, Lothgar smiles his patented, insidious grin as he glares hatefully at the woman through his narrowed eyes. "My dear, sweet, little girl. You are sooooo cute with your total lack of comprehension and naivety. Why don't you run along and play with your little dolly while we grown ups talk business. If you are a good girl and don't make too much noise, I might even give you a piece of candy." he croons in his silky, yet malicious tone of voice. If that isn't condescending enough to earn a smack in the mouth, I don't know what is.
Very nice. I like I was leaning towards a similar vein. Something like..." Oh, isn't that sweet? The cute little human girl had a thought. Run along now and find something to occupy yourself while we (insert so called superior race here) handle things."
Most of the characters are like that, but this one would not call her child, and he is a human. Oh well, idiot it is, then.
This reminds me of something that happened to me in real life. I was around 17 and getting ready to go to college. I was making small talk with an older man in a really long grocery store line while buying dorm room stuff and he was asking about my major, career plans etc. He seemed skeptical throughout the whole exchange and then he goes "I bet you'll just decide to get your M.R.S. degree instead." Not cool dude...
Put yourself in your character's place. What would you find insulting? On another thought, as long as you have people older than your character, they can always accuse them of being too young. It doesn't matter if they're a immortal demigod or a guy that's three years older.
Pet names would be insulting, at that age. "Honey," "Sweetie," "Dear," etc. What is a name you might call a child you do not know? Or you have heard a grandparent calling a doted-upon child? This sort of thing drives me crazy. You don't know me, I'm not your "honey," I'm not 16, and yes, these are my kids. And yes, I was married when I had them, thankyouverymuch! Pet names are insulting, condescending, and downright irritating. Well, when coming from someone who doesn't know me. Anyway, that is where my thought goes on it. "Girl" certainly works for this scenario as well. Looking down on the "girl" in a condescending way, but remaining more on the formal side.
