Ok, it seems like noone is following my logic here :)
I am Court
I help a lover win a heart,
You pay court to somebody to win over their favor, aka courting
I believe in chivalry,
Chivalry, often referred to courtly love, was a system of values developed and followed by the knights of the royal court
I can be juvenile at times,
Juvenile courts try cases involving underage individuals
I hang out with the bar crowd.
Bar crowd as in people of legal profession
Who or what am I?
I am Court
The title of the riddle contains a hint in a form of an important historical reference.
Historically gentlemen were nobility. Court titles often included the word gentleman.
I may have a body,
Habeas corpus - a widely used courtroom term
I wear a pony tail on my head.
Lords and judges used to wear wigs made of horse hair
People around think I am very judgemental
Courts put people on trial and judge them
I am not a brand. You all have heard of me. I may have even called you once or twice.
You might have been called to jury, or as witness
I tell people when they mess up royally
Courts tells us when we mess up. Royally is just an extra hint to help with above :)
He was/is a king, at court