Some of the best people in the field of criminal investigation are in their 70's or older and still consulted by active (younger) LE. One of the best local investigators I know is still working (and crucial in his field of expertise as no one else comes close) and he's 71.
For myself, I started doing crime scene photography for my county agency when I was 18. That counts as homicide investigation, as there were homicides (and in fact, most of what I was called to photograph were homicides of one sort or another).
I still do training for younger officers on DNA-related matters in crime scene analysis. The pre-test results are given to their superiors who typically make all of them go through the course, but select the higher achievers for further training in homicide investigation. And this is a large department with about 1000 officers, not a small department. I'd say about 5% pass the course with scores on the post-test that please their superiors (who are typically in their 50's and 60's).
Many coroners and medical examiners work up until their 80's (some still on salary, most by contract for particular cases). Obviously, people still testify to juries on these matters for as long as their expertise is sought and relevant.
Wecht, a spokesman for one of America's largest credentialing organizations, defends the company against claims that it's a diploma mill. He says the purpose of ACFEI is education and that merely having a certificate doesn't qualify a person to be a forensic expert on the witness stand.
www.pbs.org
Cyril is 91 and still active in the field in various ways (mostly as a consultant/mentor/advisor). There a