Late 30s. He'd been battling it for four years, and it was discovered at Stage 3. Which would suggest it'd been growing since when, 37 or so?
Indeed. In Chadwick Boseman’s case, the onset was at _such_ a young age that it suggests (total speculation) he might have had a hereditary cancer syndrome or some other familial risk factor. More widespread genetic testing may help people like him in the future.
I would be curious if anyone knows his cancer family history. The guidelines I've seen recommend screening at age 50 or 10 years before a the age at which a first degree relative was diagnosed (whichever is sooner).