So here's my test. I choose an icon five years older than me... and because I'm lazy I've chosen Demi Moore (because I know at 45 she's smack on five years older than me). Now, when contemplating an outfit I try to picture it on Demi - would she look like she'd raided Tallulah's wardrobe, or would she look smokin'? If flames are emanating from my mental image I'll say yes and pop it on myself.
That's my mutton high five test and I'm sticking to it.