Are the American Community Survey (ACS) 5-year estimates considered an average for the whole period or a good estimate for the middle year?

The American Community Survey (ACS) 5-year estimates are constructed as period estimates and reflect the average characteristics over the entire period. In general, unless a user knows how the estimate for that characteristic is trending over time, it is not accurate to consider the 5-year estimate as an estimate of the midpoint. However, under certain conditions, it can serve as a proxy for an estimate of the midpoint if, for example, the characteristic is stable or changing at a constant rate.

Read more about this in Period Estimates in the American Community Survey.