By convention, decommissioning should be treated as occurring at the end of a time step.
So an agent with lifetime 1 would be decommissioned on the same time step it was built.
The lifetime of an agent can be back calculated by `1+exit_time-start_time`.
See discussion here: https://groups.google.com/d/msg/cyclus-dev/jt29aJGkIdM/QqWpRpsIgvgJ The related PRs addressing this are cyclus/cycamore#300 and cyclus/cyclus#1085.
Something along the lines of: