Which population figures to believe?

I'm trying to answer a very simple question: by how much has the under 18 population in San Francisco changed since the pandemic.

The 2020 census redistricting data gives the population of San Francisco at 873,965, of which 760,738 were 18 years and over. That implies the U18 population was 113,227.

Table P12 in the Demographic and Housing Characteristics gives the same total and breaks it down by age and sex. So far, so good. 

My understanding is that the Population Estimates Program is the place to go for intercensal estimates of the population count.

I downloaded the latest vintage age/sex breakdown by county. Here's the San Francisco data for select age buckets.

The total population figure in the base year is very close to the census figure, which is what I'd expect. Notice that the 0-17 population in the base year is estimated to 118,989 whereas the census gave a figure of 113,227. That's a 5% difference which is way beyond any margin of error. 

Meanwhile, the ACS 1-year estimates are consistent with the population estimates:

Questions:

1) Why is the base year U18 population estimate so different from the census figure (118,989 vs 113,227)? Isn't the former supposed to be based on the latter?

2) Which number should I use? I want to use the 2020 census figure because then I've got a consistent series going back decades but comparing the census figure with the 2023 estimates implies that the U18 population fell by only 3,000 which is not credible for two reasons.

•  if the total population fell by over 70,000 the U18 population must have fallen by more than 3,000.
• Department of Education figures show that the number of children in school fell by about 4,000 so the U18 population must have fallen by more than that.

3) Even if we forget about the census completely, I struggle to believe the age breakdown of the population change implied by the ACS surveys and population estimates. Are we really to believe that a pandemic that killed mainly old people and led to 70,000 people leaving the city somehow caused the population 65 and over to increase by 8,500 and the population aged 75-79 to increase by nearly 25% in three years? If true, I would have expected this influx of retirees to be covered in the local media.