Well I'm of the opinion that you should wear what you want whatever your age.
You're 39 FFS, not 79...what are you supposed to do? Curl up and die in the beige section of Country Casuals?
Maybe this 'not X past the age of 30 / 35 / 40' passed muster before feminism and before we lived to reach our 80s but really 39 is a stripling.
And for all those nay-sayers, I hear there are great 2 for 1 offers on slacks at Damart.
"Style is originality; fashion is fascism.The two are eternally and unalterably opposed." - Lester Bangs (he died at 33 so maybe a bad example!)