Yes, jedi are free to leave, quit and/or retire from the Jedi Order.
As mentioned by others, Count Dooku reitred from the Jedi Order and returned to his home planet Serenno where he seized/claimed his heritage as Count.
Further evidence includes the rest of the Lost Twenty (of which Count Dooku was the twentieth): Jedi who, for reasons other than falling to the Dark Side or dying in the lines of duty, has withdrawn from the Jedi Order in pursuit of... well, anything.
Expanding our horizons further and entering The Old Republic-verse, the concept of retiring is mentioned by Kira Carsen, a companion and potential love-interest of the Hero of Tython, in the game Star Wars: the Old Republic. You find the quote here.
References other than linked
- Excerpt from RotS Novelisation, regarding Count Dooku and The Lost twenty
He is numbered among the Lost: the Jedi who renounced their fealty to the Order and resigned their commissions of Jedi Knighthood in service of ideals higher than even the Order itself professed. The Lost Twenty, as they have been known since Dooku joined their number, are remembered with both honor and regret among the Jedi; their images, sculpted from bronzium, stand enshrined in the Temple archives. These bronzium images serve as melancholy reminders that some Jedi have needs the Order cannot satisfy. Dooku had retired to his family estate, the planetary system of Serenno. Assuming his hereditary title as its Count made him one of the wealthiest beings in the galaxy
-Revenge of the Sith. Stover, M.