Running new forum is not easy these specially due to competition, 1st of all hard work then anything else
Yes you can monetize your forum very well,I handle some forums where they made $$$/month from related product advertiser.And also made $$$/month from adsense.And most of them only have 1-1,5 year old. When your forum driving targeted visitor,advertiser will give you more $$$ every month.So dont worry to start a forum site.
You also cant forget the fact that if you can get a booming forum on your hands, you will be able to sell it for a large profit more than likely. A buddy of mine has a car forum and with only 2,500 members at the time he already was getting nice offers to sell it.