The Grey Lady and the Bloody Baron are both canonically Founders-era ghosts.
Hufflepuff’s friar’s age is never specified in canon, and he could be of any era from the mid 12th century, when the first mendicant orders were founded, up to Henry VIII’s dissolution of the monasteries in the 1530s. I am inclined to believe he was earlier rather than later.
Nearly Headless Nick canonically lived in the late 1400s, based on the date of his 500th deathday party in 1992.
When Harry asks Nick in OotP about why some people become ghosts when they die, Nick tells him that only people who are afraid of death become ghosts.
It took 500 years after the founding of Hogwarts for Gryffindor to acquire a house ghost, because bravery is a Gryffindor trait.