According to current guidelines, screening colonoscopy begins at 50 years for the average-risk population, although there are not enough data on the incidence of precancerous lesions of individuals in the preceding decades. To evaluate the prevalence of colorectal polyps including potentially premalignant lesions in asymptomatic, average-risk individuals aged 40-49 versus 50-59 years, we offered total colonoscopy screening to individuals without any lower gastrointestinal symptoms. The primary end point was the prevalence of colorectal adenoma in two age groups. Of a total of 737 studies, 333 participants were 40-49 years old and 407 participants were 50-59 years old. The overall prevalence of adenomas was 11.2 and 16.4% in the group of 40-49 and 50-59 year olds, respectively. Advanced adenoma was more common in 50-59 year olds (1.2 vs. 2.9%). Malignancy was not reported in these groups. Furthermore, 77.5 and 68.6% of adenomas were observed in the distal colon in the groups of 40-49 year olds and 50-59 year olds, whereas in the proximal colon, 22.2 and 57.1% of adenomas in the groups of 40-49 year olds and 50-59 year olds, respectively, were advanced adenomas. In our study, male sex showed an association with adenoma. However, importantly, there was no significant association between age and colorectal adenoma. Although the prevalence of colorectal adenoma was similar in the two age groups investigated, the rate of advanced adenoma was higher in the group of individuals who were 50-59 years old, suggesting that colorectal cancer screening could be recommended at an age younger than 50 years.