The age range you can call women "a lady" absolutely includes thirty.
That's true ofc. But like anzuda said. Even at 30, some women don't like to be called something that is more frequently used for younger ladies, and prefer to be called something more suitable for their age.
While some prefer to be called with a term more suited for younger women. It all comes down to preference, not that it's right or wrong socially nor grammatically.
Anyway, since this is a manga, my guess is that he called her ojou-sama, which as U mentioned, is fine even if they're in their 30s. I wonder what's the term she prefers...oku-sama maybe?