Last time I was asked for my age on the web was when I created my email account. How is age helpful in user experience? Why is there an age section?
It's just another piece of information about you, same way like you can provide your real name (as the nick), your location and free text in the About section.
It does not have any "direct" value by itself, people should not treat someone differently just because he's X years old and not Y years old.
The field is optional. If you don't want to make your age public, then you don't have to fill it in.
One's age is simply a logical thing to include in a personal profile, along with one's name, location, and website. As you observe yourself in the question, lots of other services ask you to provide your age, including email providers.
Ageism is, like other "-isms", strongly discouraged.