What percentage of alcoholics do you believe developed the disease in college or between the ages 18-22?

80-90. Although, there may be no exact number of alcoholics who developed their addiction during their teenage and college years, the vast majority of alcoholics got addicted in that age range.
Vast majority < 18. The overwhelming majority (>60%) of those with alcoholism began drinking in their early teens, generally well before age 18. If people do not start drinking until age 21, the chances of developing alcohol problems are much lower, but are not zero. Much depends on the individuals genetic susceptibility (family history) as well as history of untreated trauma and other stressors in life.