Question: What is the most likely age of diagnosis of Crohn's disease (CD)?

Context:
Eighteen, 17, and 12 patients were diagnosed at ages<40, 40-59, and ≥60 years, respectively
Crohn's disease (CD) diagnosed in pediatric patients has been reported to have a more aggressive phenotype and course, with a greater prevalence of upper gastrointestinal involvement, than in adults.
There was a significant association between body mass index and bone mineral density (P = 0.004) and a significant difference in the T scores of patients according to age at diagnosis (Montreal Classification: P = 0.0006) with patients diagnosed<17 years (n = 13) having lower T scores than those diagnosed at older age groups (n = 70).
Increasing age of diagnosis was negatively associated with complicated disease and positively associated with colonic disease. As age of diagnosis increased, disease duration (P<0.001), family history of Inflammatory bowel disease (IBD) (P = 0.015) and perianal disease decreased (P<0.0015). 
Crohn's disease incidence rates in the 10-19-year age category increased by 71%, from 6.5 (1988-1990) to 11.1 (2006-2007)
Consequently, studies on Crohn's disease risk factors should focus on the population under 20 years of age.
For those with biologic use, average age at time of diagnosis of Crohn's disease was 32.3 ± 12.2 years, compared with 43.7 ± 16.3 years for those who had not received biologics (P = 0.005).
Sixty one patients (50.4%) were 20-39 years old and 43 patients (35.5%) were 40 years and older. Colonic involvement was significantly more common (46,5%) in the 40 years and older group compared with 20-39 years group (24.6%) (p = 0.01)
When the age-related incidence of Crohn's disease was plotted for all countries from which such data were available, the peaks of greatest case frequency occurred at ages 15 to 25 years and paralleled a similar peak representing the number of Peyer's patches as a function of age
Crohn's disease has a bimodal age distribution of disease onset diagnosis. The peaks (20 and 50 years) may represent different phenotypes or different genetic and/or environmental influences between younger- and older-onset individuals. 
Current therapy for CD in the UK is less likely than previously to involve the use of long-term glucocorticoids.WHAT THIS STUDY ADDS: Despite advances in therapy, short stature and slow growth continue to be encountered in children with CD.  There is a need for simple and consistent definitions of growth that can identify poor growth in children with chronic disease.METHODS: The anthropometric and treatment details of 116 children (68 male) with a mean (range) age at diagnosis of 10.8 years (4.9-15.5) and a mean age at maximum follow-up (MF) of 15.4 years (9.4-19.3) were studied retrospectively at diagnosis (T0), at 1 (T1), 2 (T2) and 3 years (T3) after diagnosis and at MF.RESULTS: At T0, mean height SD score (HtSDS) was -0.5 (-3.3 to 2.6) compared to a mid-parental HtSDS of 0.2 (-2.0 to 01.4) (p=0.002). 
White patients were significantly more likely to have ileal disease, whereas African American patients were significantly more likely to have ileocolonic and colonic disease. Age at diagnosis younger than 40 years (odds ratio [OR] 4.
Both age at diagnosis and site of CD involvement were independently associated with expression of ASCA and anti-CBir1.Compared to children 8-15 years of age at diagnosis, those 0-7 years are more likely to express anti-CBir1 but only half as likely to express ASCA.

Answer: Crohn's disease has a bimodal age distribution of disease onset diagnosis. The peaks (20 and 50 years) may represent different phenotypes or different genetic and/or environmental influences between younger- and older-onset individuals. When the age-related incidence of Crohn's disease was plotted for all countries from which such data were available, the peaks of greatest case frequency occurred at ages 15 to 25 years and paralleled a similar peak representing the number of Peyer's patches as a function of age. For those with biologic use, average age at time of diagnosis of Crohn's disease was 32.3 ± 12.2 years, compared with 43.7 ± 16.3 years for those who had not received biologics (P = 0.005).
Exact answer: Crohn's disease has a bimodal age distribution of disease onset diagnosis. The peaks (20 and 50 years) may represent different phenotypes or different genetic and/or environmental influences between younger- and older-onset individuals.