I have heard that Salman Al Farsi RA was over 300 years of age at the time he met Prophet Muhammad (S). However Wikipedia shows his birth in 568 AD. Is the report about his age being over 300 years authentic?
This is a popular idea mentioned historically by some Sunni and Shi'i authors. However, it factually seems unlikely. Many of these sources, such as Tarikh Baghdad, are not always reliable.
Sometimes these ideas come about as a way of honoring a person or to convey some theological ideas (such as the link between Jesus (A) and the Prophet (S)).
Ages ascribed to him by historians as well as narrations range from about 80 years old to 500 years old, with 250-300 years being a common account. So, it is not certain when he was born, or whether or not he had an unusually long lifespan. What can be deduced is that, at least, he was long-lived.
God knows best.