Alcohol abuse during adolescence, especially early adolescence (i.e. before age 15), may lead to long-term changes in the brain which leaves them at increased risk of alcoholism in later years; genetic factors also influence age of onset of alcohol abuse and risk of alcoholism. For example, about 40 percent of those who begin drinking alcohol before age 15 develop alcohol dependence in later life, whereas only 10 percent of those who did not begin drinking until 20 years or older developed an alcohol problem in later life. It is not entirely clear whether this association is causal, and some researchers have been known to disagree with this view.