Age Inflection Points of Colorectal Adenoma Risk in Young Adults: A Joinpoint Regression Analysis of a Single-Center Retrospective Colonoscopy-Based Cohort

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.