Absent a formal policy on extended work and flexible and phased retirement, your employer will have no stated criteria defining who can apply for what, when. Because there are employers who have designed and implemented successful initiatives along these lines, some shared standards have emerged that might provide some guidance.
By definition, it has to be a subjective decision on your part when it is time to rethink your career path. For some, there will be subtle or not so subtle hints that it might be “time for you to look around” for other options. Frequently this can happen in one’s early or late 50s – and in some industries, even earlier. The time to consider the options covered on this site is simply when internal and external signals point in that direction.