Well, using that logic, I'd rather cast a 30 - 40 year old woman to play a younger role (if she is believable) BECAUSE the younger women get more chances. They can always go be in Spring Awakening or RENT or something (evil chuckle).
But seriously, it's a matter of what's easier for the actress in question. Younger playing up, or older playing down. We can't accept or dismiss someone based on their age alone. It's really it's a case by case basis. The last time I saw Phantom (about 7 years ago) the woman playing Mme. Giry was only about 35, but she had a heavy voice that made it easy to sound like an older woman, and she was bony, which made her look old. It was just a matter of her make-up and carriage...and boom...older Ballet Mistress.
Someone said and "older" woman (by 'older' I mean over 35) is more likely to be able to relate to a character that grows and matures because she's been through that in real life. Not like someone who is 21 or 22 years old and still finding themselves. I think it's true. Not saying a young woman can't do it. I hear Fantasia is really good at it...but then again she's been through a lifetime's worth of drama in 22 short years, so maybe she isn't the best example. But you get my drift.
And since when is 30 - 40 considered "Middle Aged"? Yuck!
"I'm-Not-That-Boring-Low-Ass-Girl?! You better go up at the end!" - Seth Rudetsky to Julia Murney about her Solo CD choice