Age Inflection Points of Colorectal Adenoma Risk in Young Adults: A Joinpoint Regression Analysis of a Single-Center...

Background/Objectives: Early-onset colorectal cancer (EO-CRC) incidence continues to rise globally, yet age-stratified risk data for adults under 45 remain limited. Methods: This retrospective, single-center, colonoscopy-based cohort study included 3959 examinees aged 18-44 at Peking University International Hospital (2023-2024) and used Joinpoint regression to identify age inflection points for polyp detection rates. Case-control analysis with Least Absolute Shrinkage and Selection Operator (LASSO)-penalized logistic regression assessed metabolic risk factors for adenoma and serrated polyps in the 40-44 stratum. Results: Detection rates for polyps, adenomas, and serrated lesions all rose with age (p < 0.001). From the youngest (18-29) to the oldest (40-44) group, the polyp detection rate increased 2.6-fold, the adenoma detection rate (ADR) 4.3-fold, and the serrated lesion detection rate 2.0-fold. Joinpoint regression revealed an ADR inflection at age 33 (annual percentage change (APC): +0.51% → +1.27%), a high-risk adenoma (HRA) inflection at age 39 (APC: +0.14% → +0.77%), and an advanced-neoplasia inflection also at age 39 (APC: +0.20% → +0.81%). Across age strata, adenoma and advanced-neoplasia detection rates were comparable between asymptomatic screening and symptomatic examinees from age 35 onward (40-44 ADR 20.5% vs. 21.6%), whereas symptomatic examinees had higher rates in the youngest strata. In exploratory cross-sectional analyses within the 40-44 group, total cholesterol (odds ratio, OR = 1.47) and body mass index (BMI) (OR = 1.05) showed associations with adenoma, without establishing causality, with BMI elevation conferring risk only in women (p for interaction = 0.018). Conclusions: Adenoma risk accelerates at 33 and high-risk lesions escalate at 39, providing age-stratified risk benchmarks for adults under 45 in a predominantly symptomatic Chinese cohort. These inflection points warrant prospective validation in asymptomatic screening populations.