Coming-of-Age
Usually in a Coming-of-Age story, the main character moves from adolescence into adulthood. But in The Ambassadors, Strether's goal is to recover some of the sense of possibility and wonder that he has lost since his faraway youth. Early in the novel, Strether worries that he has let life pass him by, and he spends the rest of the novel trying to experience everything that he never got to the first time around (within limits, of course—this is a respectable gent, after all).
The reason Strether hasn't experienced these things is because he has become set in his ways, and his journey through the novel is centered on how well he can manage to break out of this habit and rediscover some of the open-mindedness that he sees in the younger people around him.
And it's only when Strether can make the unsafe, impractical decision of breaking things off with Mrs. Newsome that he truly begins to start life anew and to live each day to its fullest. Now of course, James complicates this at the end by having Strether decide to return to Woollett. But still, the bulk of the story's development is based on Strether's rediscovery of his youthful open-mindedness.