Having had long hair for 20 years (before I cut it last summer) I know that it gets really tiring to be constantly told to "cut it for a change!" Personally I LOVE LOVE Namie's long, amazing hair. And she should keep the miniskirts and the boots. Personally, when I think of Namie, I think of WANT ME, WANT ME Namie, and that encompasses all those things.
Sometimes you just know what works. Plus she's well into her career enough that she has control over her image, much like Ayu. Personally I think Ayu looks better with short hair, but I know she'll keep going back and forth back and forth, because that's just her style.
And I don't really believe in "age appropriate" in regards to "too old". If you look good, you look good. And Namie is HARDLY too old for miniskirts. She still looks younger than *me*, for goodness sake, and I was born in a later decade than her.