Ditto to all of the above with complications being the biggest risk factor.
Specific to us. My daughter went undiagnosed for at least 18 months, with the 6 months preceding her diagnosis being the worst. She received her diagnosis on the operating table and even then they only went in with the expectation of removing her appendix and just having a bit of a look around. Turns out she had a ruptured bowel, pus throughout her abdominal cavity and peritonitis. She came within a whiskers breadth of losing her life. Again I will stress she was undiagnosed at the time but also her bloods didn’t reflect the severity of what was happening and nor did an ultrasound and CT Scan, she was not scoped and her symptoms were not what many consider ‘classic’ for Crohn’s.
My son was diagnosed very quickly but he also quickly developed complications. Fortunately for him we knew what we were dealing with, plus his bloods were very sensitive to changes in his body. However, even with prompt hospitalisation and treatment for perforation, fistula and abscesses it was repeatedly stressed to me how sick he was (I did realise this without being told) so I guess failure to respond to treatment would have made his situation very dire indeed.