I might guess that people list their ages because they feel it gives an indication of the resources they have available to them. I might guess that someone who is 17 is still in school, can't afford to buy a high end computer or compiler, may be thinking about their career path and what colleges or university to attend, and has many years ahead to perfect his trade. Where as a person that says he's 36 probably already has a job, might be able to afford more equipment, might not be planning to go back to school to learn programming suggesting he might be more interested in books or online material than a younger person. All sorts of stereotypes you can draw by a person's age and it's easy to communicate them by simply saying, "I'm 17."
That is very true. But I, like many other young ones coming it to the world of game development seek help and guidance so we don't end up in a dead end job making the next My little pony game.