Not in the habit of getting c-peptide tests but I can say after 30 years with the condition (roughly) there was a sudden massive difficulty spike in keeping levels under control, with every little thing bothering me in a way it never had before. Coupled with this, I found the immediate effects of having higher than target Blood Sugar became much more pronounced.
Basically, I could do roughly 80% in range without any real effort in the first 30 years but all of a sudden it to gargantuan effort just to hit