Classmates of mine who were on the young side often had brand name work experience and big name undergrads. They usually were the ones who you would see as being on the MBA timeline from the age of 18. Went to a top 10 undergrad, worked at MBB or specialty finance area, then an mba after 2 years. Work experience is much more valuable than many younger applicants realize. Lots of companies have a preference for more mature and experienced workers.
You can take the GMAT, its good for 5 years but I suggest getting at least 2 full years of work experience before applying...and thus having 3 by the time you start. Even then another year or so might be helpful. Honestly most of the resumes I have seen for people with only a few years work experience are lacking, they dont have the leadership stories and aren't nearly as compelling as they would be with another year or two.
Kellogg Class of 2010...still active and willing to help. However, I do not do profile reviews, don't offer predictions on chances and am far to busy to review essays, so save the energy of writing me a PM seeking help for these. If I don't respond to a PM that is not one of the previously mentioned trash can destined messages, please don't take it personally I get so many messages I have a hard to responding to most. The more interesting, compelling, or humorous you message the more likely I am to respond.