Ensign Flandry And Cadet Loftus.
Coming of age is a major theme in juvenile fiction. I just remembered two passages.
"She enjoyed the rest of the voyage, even after she had identified the change in him, the thing which had gone and would never quite come back. Youth."
-Poul Anderson, Ensign Flandry IN Anderson, Young Flandry (Riverdale, NY, 2010), pp. 1-192 AT Chapter Fifteen, p. 152.
"When the long jump from Ss'pode to Malis had begun, Jack had been on the down side of eighteen years old. When the alarm exploded and the Argo came off the Standing Wave, he had turned twenty. But it felt much more like eighty."
-James Blish, Mission To The Heart Stars (London, 1980), Chapter Eight, p. 92.
Both Dominic Flandry and Jack Loftus come off age on long interstellar journeys.
Ensign Flandry is Volume I of the Young Flandry Trilogy, part of the Dominic Flandry series, which is part of the History of Technic Civilization.
Mission To The Heart Stars is Volume II of the Jack Loftus diptych, which is part of the Haertel Scholium.
Addendum: In SM Stirling's Emberverse, pages become squires become knights. Coming of age is formalized.