When do all races get into a common town?

Solution 1:

Lion's Arch is the game's central town. It can be reached from each starting city through an Asura portal. You can also get there through the Mists by transporting yourself from the hero panel's PVP tab.

If you choose not to use these methods, the personal story quests will have you walking there by level 30.

Solution 2:

As soon as you're out of the opening instance, you can all meet up. After you've completed your tutorial instance (the tip-off is fighting a rather large boss in each instance), you'll be standing a small distance in front of your home city. Inside that home city there will be a asura gate to Lion's Arch, from which you can access any of the other home cities.