Look at the way Smitty and Kurt dress during the pitch meeting with Don and compare it to the of the older members of the SC staff.

Goddamn kids and their music.

Funny I noticed the same thing--their clothes and how casual they were dressed in comparison to the rest of the guys!  They may be the  vanguard of the new generation in advertising.  I liked how they reacted to Don's criticism too-no big thing, they'd try again.  Peggy, however, appeared a bit nervous in that meeting and man I'd love for her to ditch the plaids!

Sure they dress differently and sure they are younger - but as season 2 took pains to point out, they themselves are really only token hires in the company scheme of things - a gesture on the part of C&S towards trying to figure out what they young want. Hired mainly for their age rather than their experience in advertising, their dress, language, and attitude is as much a uniform as the suits of the jr. execs or the secretaries fashions. The cohesiveness of the incredible pressure of this society at this point in history is represented so realistically we see how no one is immune from it - everyone is playing a role right down to the "non-conformist" youth.
It's fantastic.