I would say 51.
I might be superstitious, but I think once you can work on AA's you see a rather large downscale in the amount of exp you get per kill.
Sure, level 69 takes a while, but in my opinion the biggest smack in the face is at 51. People probably just don't notice it as much because
at 51 you can still get a hefty PL from a level 70 up until maybe 60-62 range. After that, the 70 sees too many
Lt. Blues to move your exp bar (fungus grove anyone? Lol)
I believe they develop the progression of the game in balance with what a character is "capable" of, like at 51 if I really wanted to, I could get a few hundred aa's to sink into extra stats. Thinking back to say Luclin (were aa's around then?), a +5 to Str was a big deal wasn't it? So thereby making a respectable difference in the players abilities at that level. That is worthy of curbing exp (for the devs) so power doesn't come easy.. .. . and then some a-hole made Macroquest. I swear if I ever meet a person using that hack program in RL they'll have reason to take me to court! F*in ruined EQ.. but I digress.
Don't think of the game as what level you are.. Think of it as "Do I have enough AA's for my class at this level?" and then ask, what is half of that?
Just kidding man, valid question indeed, and very interesting about the 51-60, 61-70 same as same as stuff Brudah! Good thread.