I think it is useful to know the age of posters before replying. It can make a big difference if you are replying to a 14 year old girl living with parents or to a 60 year old woman, especially regarding any "sensitive" topics or making any jokey innuendo.
Perhaps one thing that puts some people off putting in their age is the fear of phishing? I was initially reluctant to put in my real date of birth for this reason, so in the end opted for putting in a date in the region of my true date of birth.
Perhaps there should be an option to only enter just the year of birth? For most older people the display of age doesn't need to be precise.
Perhaps another solution would be to display a range of age e.g. <18, 18 - 29, 30 - 39 etc?