WHAT IS YOUR PATRONUS?

The Patronus Charm is one of the most ancient defensive charms and appears in many accounts of early magic. The aim is to produce a silvery-white guardian or protector, which takes the form of an animal, against the magical creatures called Dementors.

Nobody knows. The first account of Dementors can be found as early as the 15th century.
Azkaban, the infamous wizard prison which was originally the fortress of the dreaded Ekrizdis who practised the worst kinds of Dark Magic, was already infested with Dementors prior to Ekrizdis' death at the end of the 15th century. We have no recollection of Dementors prior to that. The great eighteenth-century researcher of Charms, Professor Catullus Spangle, set forth certain principles that are widely accepted as true. If it was well understood by the 18th century, then of course it was likely invented long before then, smetime between the 15th century and the 18th century.

It is one of the most powerful defensive charms known to wizardkind, and the only defensive charm known to be effective against Dementors. It can also be used as a messenger between wizards.

Yes. The Patronus Charm is a Highly Advanced Magic, well beyond Ordinary Wizarding Level. Students of Hogwarts learn this charm in their 7th year.

Yes. The form of a Patronus may change during the course of a witch or wizard's life. Instances have been known of the form of the Patronus transforming due to bereavement, falling in love or profound shifts in a person's character. A Patronus is unique to the caster and is very personal. Each person's Patronus is therefore different in some way.