It is xactly 20 lvls. I had a friend lvling a healer while I was lvling my warlord. She was grouped at 19 when I was 40 and wasnt getting much xp, The xp Jumped dramatically for her when she hit 20 and I was still 40(it took about abother 4 pulls for her to hit 21 and ditto for 22, but thats another story.)