Sounds like is isn't actually a command in events, but a parameter of the create_* commands.
The thing is, marriages, divorces, gain/loss of titles, and various modifiers happen in history files, and are marked as happening at the correct times. Applying that to traits should not be difficult. Adding immortal_age to history is a kludge, and it's not even clear how it would work. If the game hasn't added their traits yet before game start, then immortal_age is meaningless at the history date you would need to set it. So making it available in history would *still* require the same dev effort as a new history command. And it might as well be a new general command, which could incidentally be used in history. *That* would open up some interesting possibilities (like a form of immortality in which one ages, just slower, or one in which apparent age actually reduces!).
A new command doing this could be interesting I would admit, But I would sooner see the existing toolset simply be made to work so as to handle where we currently are. Otherwise we need to define multiple portrait sets as it is to better handle the issue in question.