As Crater says, there's no specific level that leads into a 'plateau' effect. Essentially, every level gained 'slows you down' mathematically the same as the previous one. It's something like, using example numbers, exp required for the previous level x1.05; So at level 1 you need say 10,000 exp to reach level 2; But to go from 2-3 would require 10,500. And then 3-4 would take 11,025, 4-5 would take 11,576, and so on.
Not sure what the exact multiplier is off the top of my head, i think the base exp required originally is 10,000, but that's essentially how it works.
That said, You'll kinda notice you're working for levels around 125-150, just because the exp required is getting rather monumental.
And you'll max out all your perks at 276 i think, with a max level of 300.