The only "important" reason is the religious one but there aren't really any reason to circumcise unless it's for medical reasons.
It is not. Less then 20% of the words men are circumcised and most of these for religious reasons some ostensibly for outdated reasons of health benefits.
When you start puberty and forward. You have usually reached the finished size at 15.
He has 2 - circumcision and Bar Mitzvah.
Circumcision, Dietary Laws, Temple.
It isn't important at all on the contrary circumcision causes reduced sensitivity which cause some men to have to give up before ejaculating. it also causes vaginal dryness problems discouraging the female partner.
No, the penis is not shortened by circumcision.
No circumcision is not beneficial to health. on the contrary it is harmful.
No, penis size has nothing to do with circumcision.
Circumcision has nothing to do with infertility.
Circumcision is painful and only necessary if you don't wash very often, so its understandable why the tradition started among the Jews and Muslims in the desert - there was just not enough water there to keep it clean. However in a developed society damaging the penis by chopping of an important part of it is ill advised and any responsible doctor will try to talk you out of it.
When you develop SERIOUS conditions where circumcision is the Only option.
The circumcision scar is permanent and it can not be gotten rid of.
No, it is nothing to do with it. Circumcision is the removal of the foreskin from the penis.