At what age does a Sr. become a Sr? Is it when you name the Jr.?

I received a college alumni newsletter in the mail yesterday, and something I read triggered a question.

Someone (no, this is not his real name) was mentioned as “Joseph Antonelli Sr.”. Now, the guy was born as Joe Antonelli (no <senior>).

My assumption is that when he and his spouse had a male child, they named it Joe Antonelli Junior, and that’s when the dad decided to assign himself the “Sr.” affectation.

Is that what normally happens?

Why, in our enlightened and egalitarian society, do women not have “Junior” or “III” as name pieces?

Is the whole Junior thing just a huge ego boost on the part of the parent?