What season does Blair start having an eating disorder and who is the first to find out????

She have scince before the season 1 start. In 1x09 flashbacks, her mother talk about it like it make a long time so.

She has it since before season 1 (not really sure her age) and I think her mom, Dorota and Serena know.
As for the others I guess they knew about it, but Eleanor, Dorota and S are the ones I'm sure know about that.