"The glory of young men is their strength, gray hair the splendor of the old" (Proverbs 20:29). Not too sure if I'm ever going to have gray hair...since I don't really have any, but I think this scripture is alluding to the experiences that resulted in his gray hair (or lack thereof in some cases).
Honestly, what man doesn't want to be considered a Sage when he's old? To me, that's one of the rewards of overcoming the trials and tribulations of life. Moreover, there's a responsibility that the elders of our society have to the younger generations, so we can learn from their experiences and try not to repeat the mistakes of their past.
Eldredge only allocates one chapter to this stage in life, mainly because he admits that he's not able to provide that much insight since he's not experienced much of Sagehood. All the secondary chapters devoted to a stage focus on how to develop a man through this period in his life, yet the Eldredge chooses to only devote a section in this chapter to the aspect of raising a Sage. Although you can't exactly raise a Sage, it is possible to raise the Sage in a man. For instance, the author takes note of how an older man can be undeveloped. This occurs when he either refuses to take the journey, or take note of his journey. If he has failed to take stock of his experiences, then all we could learn from him is lost. However, if we can draw out those experiences through communication, or simply listening to him, we can bring out his words of wisdom. What is more, when I think about the term wisdom, I can't help but think of King Solomon, the man who asked God for wisdom, and was granted it. Arguably the wisest man to ever live (aside from Christ, of course), he wrote that "Fear of the Lord is the beginning of wisdom" (Proverbs 1:7). This scripture delineates between mere knowledge and wisdom. In my view, man gains knowledge by what happens to him, but he garners wisdom by how he reacts to those experiences. So, when judging the Sage-ness of a man, one must look at how he reacted to those situations he faced throughout his life. By doing so, you can determine whether or not to follow in his footsteps.