Thanks for your speedy reply. 
That sounds similar to what I’ve been doing, so I’ll keep at it.
Warning: ramble ahead
We have use cases for this in the project management world, too. Default in that world is, indeed, as Jess stated, that duration remains static. But, of course, there are those times when a date on one item gets moved, and the duration of another item is impacted to be shorter (usually, and often meaning long work days and/or overtime to compensate) or longer (ha!).
And it could certainly apply within the genealogy world, as well (which I thought you were describing until I re-read your initial question. 
fellow character-wrangler).
I have scenarios like yours, where I know the age and would like to have the start date calculated and have any adjustments by a constraining item (e.g., death event) keep that person’s duration static by adjusting the date for start (prior event) or end (subsequent event) accordingly. 'Cause yes, we work in the future, too, right?
Then I have scenarios like I described above with project management, where it would be best if the duration could be adjusted. I’m tracking data points for a novel wherein people might continue to exist after death and where reincarnation and immortality are possible. To handle this insanity, I add events for birth, death, and any rebirths or subsequent deaths. Then, I created new event types of lifespan and afterdeathspan. I constrain those new types to the start/end of the birth, death, and rebirth events for the associated person. Then I just keep manually adjusting each Person record to reflect birth date (first), duration, and death date (last, if applicable) as needed. I’d much rather be able to adjust the events constraining the overall start and end of the Person and have the duration of that person auto-adjust accordingly.
All-in-all, I think it would be great to be able to lock any one of the start/end/duration combo items in an event as an alternative to the current option of locking all three of these at once. So, we could pick duration on one, end date on another, start date on yet another, and all (current option) on still another. Such an implementation might make resolving conflicts a little more hairy, though, so maybe it could be an “advanced” option in settings that someone has to intentionally enable with an appropriate use-at-own-risk-of-frustration disclaimer. 
