You can apply to all top 20 b schools in the USA, even though they state that the median job experience is around 5 years, they welcome younger applicants with good track record.
I would suggest that you dont worry right now, about the schools you can apply, as people keep applying as late as november. But if you want to be safe i would suggest you apply as early as august, september, one thing you need to remember is decide what you really want to do as you will be spending approx 100K$ and if the knowledge is of no use after your graduation then you would end up regretting. So the most important thing you can do in the meantime when your working on GMAT is decide what you want to do and why, how it can help you.