I think that for someone to commit suicide it means that his/her whole spirit was broken to such degree that he/she can't do anything to fix things. Therefore, they believe that all they can do is commit suicide, which even then they find difficult to commit(actually everyone would find it hard to do).
I guess that they must be able to break through the human's self-defence kind of system that stops us from harming ourselves and go with it. personally, I think that they don't realize the outcome of it. They think that if they are gone, so will be their problems and since "nobody loves them" nobody will mind. What they fail to understand is that after the deed is done and they are "free", those left behid (friends, family) are those who feel the real pain.
As for the age, I think that teenagers and yound adults do this shit because they feel weak and that the world is too cruel, whereas older people have endured hardships that makes them stronger and gives them the ability to survive, with exceptions of course. :)
I know, tl;dr but meh, deep questions, long answers. and sory if someone worte something similar the same before but I was kinda bored to read through all of the posts :D