Is there any dead carbon, like perhaps in the teeth? Have never heard of it being done, but would be neat.
Not 100% sure, but for someone that just died, you might be able to do it with their brain, as brain cells are hardly ever renewed. For a living human I don't think there is any way that's non-invasive.
I recently tried to read up on the topic and everything I found stated that brain/nervous cells are generally barely regenerated and can stay with you your whole life. Granted, most articles trace back to this source[0] from 1967, but if newer information exists, they've done a really bad job at spreading it.
[0]: https://bionumbers.hms.harvard.edu/bionumber.aspx?&id=101940