That is an excellent question.
A mutton is so only because and as long as it is compared. In this case, compared to its baby version, the lamb. The mutton could be anything; But the lamb serves as boundary. An imposed boundary disguised as a natural one (the distinction between lamb and sheep is one of nature: a lamb grows into a sheep, but the distinction between lamb and mutton is not so natural anymore: a lamb grows into a sheep which latter becomes mutton stew thanks to human intervention). The terrible "Mutton dressed as lamb" image is in fact constructed. It's cultural, it's social. There is a purpose behind this idea. It is a scary, degrading message meant to secure you, post-twenty-sometheling, into non-lamb territory. We wouldn't want you to threaten social order now would we.
Think about it. Whenever this image is conjured, we're talking about looks that are wrong for all kinds of reasons, however could possibly be excused on younger wearers because their youth could make up for a lot of things.
It's about what is in the head of the onlooker, not the wearer.
Here is a description I found on the internet: http://www.galleryoftheabsurd......-lamb.html
the outfit depicted here isn't wrong because of the age of its wearer. It's wrong because... come on! The visual given by the blogger is just terrible, even for " a 20-something heading to the hottest new club"!
So my answer to you is: in fashion, and in style, there is no mutton. There is no mutton for the simple and good reason that there is no real lamb. Remember this.