-- Young age at first drink. Perhaps because of changes in the still-developing brain and because they associate with peers who are also heavy drinkers, people who drink at age 15 or younger are at particularly high risk of developing an alcohol problem. The national survey found that nearly half of people who become alcohol-dependent do so by age 21 and 75 percent by age 25. -- Flushing reaction to alcohol. Some people carry a gene mutation -- ALDH2 -- that affects alcohol metabolism and causes them to turn red when they drink. Seen mostly in people of Asian descent, the gene is linked to a higher risk of alcohol-use disorders but, conversely, the uncomfortable flushing effect often dissuades these people from drinking. -- Low sensitivity to alcohol. Some people need to drink more to feel an effect compared with the typical person -- often referred to as the ability to 'drink everyone else under the table.' At least five genes are thought to be linked to this proclivity. An estimated 40 percent of children of alcoholics carry this trait. -- Specific gene mutations. They include a mutation of the 5HTT gene, which is linked to low serotonin levels in the brain and is found in 60 percent of people who were alcoholics at age 40, and a mutation of the GABA (A) gene that is linked to a low sensitivity to alcohol. -- The presence of behavioral, emotional or psychiatric disorders -- or smoking dependence. Any of these factors, when combined with a pattern of exceeding drinking limits, should help people gauge their personal risk, experts say. 