Set the starting group, let some years pass, apply an event, let more years pass. The solver keeps the total and the head count in separate stat tiles and divides only in the last step. The first three cases are the same couple over the same seven years with three different events — and three very different averages.
Couple averaging 28; a child born after 2 years; measured 5 years on
Total 75 · average 25 · started at 28
Compare the first three tabs: identical family, identical seven years, averages of 25, 33 and 35. Nothing but the event differs. To see a guard fire, ask for a 60-year-old to die out of a two-person family averaging 25 — there is not that much age in the household.