style icons are people who influence style.
Mary-Kate Olsen influenced Marc Jacobs and many other top designers mentioned the olsens.
Have you not seen the catwalk lately? All of the top designers have outfits that are screaming MK. Whether you think they have style or not, you can not deny it, these girls have influenced fashion. From the catwalk these looks are watered down to eventually filter into mainstream chain stores.
But i agree they are too young to be considered style icons. To say that any of the "it stars" are style icons is absurd. How can you say this after a couple of trendy seasons.
I do beleive that MK&A Olsen will stand the test of time.
Also i think Ashley differs from the bartons,richies and lohans because she shows youth but MATURITY in her clothes. Something the other brats are largely lacking. (Sorry to offend anyone by calling them brats).
When you see girls shopping in the city, you can tell the ones that are just rich and can afford to buy from designers and follow trends, and than you see the ones that actually have their own style.