Although you can level to well over level 70 in Outland you should consider moving to Wotlk zones at around level 67 – 68 for MUCH better XP. The XP here is better and questing and leveling is a lot quicker.
Howling Fjord Level 68-72
Along with Borean Tundra, the Howling Fjord is the starting zone in Northrend. Both zones are viable from around level 68 and combining questing in both zones can be very efficient.
Borean Tundra Level 68 – 72
Borean Tundra is one of the two starting zones of Wotlk and is one of the first areas you will visit. Go here at around level 68, then move on at around 72.