Frisian dates back as far as the Early Middle Ages. It began to become a language distinct from other North Sea Germanic languages, such as Old English.

During the High Middle Ages, Old Frisian was used as a written language (e.g. in legal texts) and as the official regional language. During the course of the 16th century, Frisian practically fell out of use as a written language. The 19th century saw a return to Frisian as a written language and a vibrant literary climate emerged.

Source: Taal fan it Hert, brochure

More information about the history of Frisian
An introduction to Old Frisian (book in Afûk webshop)
The Story of Frisian in Multilingual Friesland (book in Afûk webshop)