I've been duoing on my sorcerer since level 12 with a friend. We actually have the problem of starting the planets too low of level. We clear all the quests, bonus, and most heroics on each planet, and we still start planets 2 levels before their recommended level. We started Corellia last night at level 45. She's a tank and I'm a healer so it's not too bad, but fighting level 50 elites at 45 gets a little rough occasionally.
Now on my Juggernaut who I am leveling solo.. I'm almost 44 and have done like 20% of the quests on Belsavis.
I'll most likely be 46 when I finish this planet up giving me a large lead on the next planet.