I'll second that.

Move to Outland at 58th.
Move to Northrend at 68th.

Going earlier doesn't make much sense, in that the mobs are too high to hit consistently and they really pound you hard. At 58th/68th, you can take the quests, and Outland loot is a huge boost over Classic wow, while Northrend loot is as huge if you're just leveling.. although not so much if you have Outland raid gear on any of the toons.

With Borean Tundra and Howling Fjord, you can do one at 68th and the other at 70th, so be on equal footing as far as leveling pace to someone who went to Northrend at 70th. Grizzly Hills or Dragonblight after those two. Instance Boosting or Alterac Valley are options if you don't like collection quests, which are plentiful in Northrend. Still questing in Northrend is probably faster then boosting or pvp, but it can be more painful too.