大般涅槃經疏 12.32

12.32

「復次如人生子」下，是品中第十二，答上云何說法者增長如月初之問。前舉月問佛，今約生子悉有增長答之。十六月者，二解：一云十箇六月，即是五歲；二解始經十六月，即是過周四月。在文可見。

From "Furthermore, just as when a person gives birth to a son" onwards is the twelfth [section] within the chapter, answering the preceding question, "How does the expounder of the Dharma wax like the beginning of the moon?"
Previously, the moon was brought up to question the Buddha; now, it is answered in terms of a newborn son all having growth.
Regarding "sixteen months," there are two explanations: one says ten periods of six months, which is five years; the second explanation is having just passed sixteen months, which is four months past a full year. This can be seen in the text.