I was in PT 1 and I'm lvl 50 and everyone was at lvl 30 so far. What you could do is play it and if their to strong you can make like a nomad and "JUST LEAVE!!!" --- LORD_HOGAN "He who does great things does little to prove it."
Level 15 is the earliest you should start them. It will be that level when you get there if you go at 15. Quests scale with you up to the 30s. --- http://img.photobucket.com/albums/v643/ripvanwallace/SCAN001.jpg
I was in PT 1 and I'm lvl 50 and everyone was at lvl 30 so far. What you could do is play it and if their to strong you can make like a nomad and "JUST LEAVE!!!"
I also found that every mission (except for the Tier 3 battle missions and the "Pete the Invincible" mission) capped at lv.30 on Normal mode. The Tier 3 missions (including the Tier 3 rematches) seem to scale to your character's level, from what I can tell (my lv.50 Siren was getting these as lv.50 missions ON NORMAL MODE), and the "Pete the Invincible" mission is obviously lv.50, seeing as it's effectively a raid boss mission. --- "Don't eat me. You won't like me if you eat me."
i say do it before the final assault on jack should be about lv 30 then and will give you a nice lv boost to take on the big bad robots --- I just wanted to raise his affection, NOT have him lick my nipples