Why does the Torah see fit to tell us how long Yishmael lived?
Rashi: To learn from it that Ya'akov must have spent fourteen years in the Yeshivah of Eiver, after leaving his father's house, before arriving in Charan. 1
Ramban #1: It tells us about the children and the life-span of Yishmael, to teach us the extent to which the sons of Tzadikim are blessed. 2
Ramban #2: Because he did Teshuvah and is considered a Tzadik, the Torah writes about him like it does about Tzadikim.
Rashbam: In honor of Avraham, since it informed us of his birth and his Bris, it also tells us how many years he lived.
Why does the Torah write "va'Yigva va'Yamos"?
Ramban (citing the Midrash): Following stomach trouble, which is the classical death of Tzadikim.
QUESTIONS ON RASHI
Rashi writes that the Torah writes the years of Yishmael 'Kedei Leyaches' Yakov's years. Why did he not say 'to know Yakov's years'?
Ha'Emek Davar: This teaches that his best years were when he learned in Beis Ever without any distractions. This teaches us that one who can learn now and marry afterwards should do so, and not say 'my time [to marry] has come.' 1
Kidushin 29b says that sometimes it is better to marry first; it depends on whether one is in Eretz Yisrael or in Bavel. Perhaps ha'Emek Davar discusses one who will not be able to learn after he marries. (PF)