[natpalmer1776]

Personally, I would rephrase it to:

Ann is 6 years younger than Mary, who is 24 years old.

Or even better, Ann is 18 and Mary is 24.

But then again, I always disliked word-based logic problems that didn't correspond to genuine complexity. If something CAN be stated in simple terms, I believe it should be. If you want to create a word-based logic puzzle then present a genuine problem that can be engaged with rather than a poorly formulated explanation of the mundane.

That being said, I have no idea how I would present the same fundamental logical puzzle using a material problem that doesn't have a simpler distillation that solves the problem for you. Maybe I'm just not cut out for the realm of intellectual puzzles.