Well I think for many departments/companies, minimum age is 18 but you need relevant education and experience which would be difficult to obtain before at least 22. Unless you attend University at a young age.
We post a lot of job opening announcements on the employment page of this website. Very seldom is a minimum age included in the announcement. If you have an idea of were you might want to work you can call their Human Resources Department and ask what they require.
how old do i have to be to start my career of being a crime scene investigator?