A lot depends on your overall dental history and routine dental care and oral hygiene, your diet, whether you smoke or not, the afftects of chronic use of certain medications and hereditary factors - even if you had braces at some point. For many it can start in the late 40's or early 50's or even later but it can start in the 30's or earlier as well depending on the aforementioned factors. I wouldn't worry too much about when its going happen but instead focus on preventive measures so it doesn't happen.