I find the whole idea depressing, because I suspect that my house is telling the world I'm 80.
But let's explore the idea and apply it to clothes. Can people tell your age from your clothes? I think they can.
Young people just starting out stay current because current is all they know. If my daughter decided to buy a white t-shirt, a jean skirt, and a cardigan, she'd end up getting something like this:
Jean skirt from The Gap
Meanwhile, my mother would end up with stuff like this:
T-shirt from Lands End
Now, when you think about it, nothing says a cardigan has to be a plain, button-up, long-sleeved sweater. Or that a jean skirt has to graze the top of your ankles. Or that a t-shirt has to be boxy.
And certainly nothing says you have to wear the old lady stuff from head to toe.
So for today, do a youth inventory of your clothes. Figure out the likely demographic for each piece you're wearing. See if you can't tweak your look a little. (And include your accessories! The right shoes, bag, and jewelry can shave off a lot of years.)