[Avodah] How to become a kohein

[Sholom Simon via Avodah]

><<< But avraham was *not* past fathering children at 99.  He fathered 8
>more when he was in his late 130s/early 140s, and there's no indication tat
>this was any kind of miracle or surprise. >>>

Don't we also have an old-age issue re: Yocheved (bas Levi)?  If she 
was born "between the gates" then she was 130 (?) when Moshe was 
born.  I vaguely recall that ibn Ezra (?) used that argument to prove 
that she was not born between the gates.  But if she was born 
significantly after that, then (naive question) how old was Levi when 
Yocheved was born?

In short, I guess I'm asking: how many years passed  between the 
births of Levi and Moshe?

-- Sholom