Day one is the business day for "real" press (though I've heard it's rather easy to blag your way in) and industry folk, while days two and three are open to the public (and obviously no age restriction there).
Nice! I've been wanting to visit a gameshow lately, but E3 requires a job in the game industry, and over 18. But TGS I suppose would be much better. I think I'll try to go in 2005.