[Avodah] basic parsha question from Chaya Sarah -- Yishmael's age

[Sholom Simon via Avodah]

Shavua tov,

Much ado is made of Sarah's age as described in Torah (100 years and 
20 years and 7 years).  But at the end of the parsha, the same syntax 
is used for Avraham and for Yishmael (100 years and 30 years and 5 
years).  I *did* see meforshim explain why his age is mentioned at 
all, but I didn't see why it used "shana" three times, like it says for Sarah.

Thoughts anyone?

-- Sholom