For starters, Dodong thinks that the age of seventeen is not young anymore, although he realizes that his father is likely to disagree with him on this matter. To his way of thinking, the fact that there are pimples on his face and that his mustache is growing dark mean that he is no longer a boy, but now a man. This feeling of being an adult is empowering and, coupled with a growing focus on his virility, he feels that it is time to marry.
His desire for "his girl" Teang is another reason that Dodong thinks he has reached the right age to get married. He does not want to wait for a physical relationship with the young lady who leads him to "dream even during the day."
When Dodong tells his father of his plans to marry, his father makes the point that seventeen is a young age at which to marry. While his father grants his permission, Dodong fails to notice the "strange helpless light" that appears in his father's eyes.
Over time, and with the birth of many children, Dodong and Teang both realize independently that in fact, they married and started having children far too early. When their son, Blas, reaches the age of eighteen and comes to Dodong and expresses his own wish to marry, Dodong has the same reservations that his father had all those years earlier.