Does PostgreSQL consider how evenly data is distributed on disk when choosing a seq scan?
In short, No.

One of the statistics which PostgreSQL collects is the correlation between the values of a column (actually the ranks of those values) versus the location of that row in the table.  You can find that result in pg_stats.correlation for your specific tablename and colname.  

However, the particular query you give is not one of the situations where this information is used.  The primary use is to estimate how much of the IO done for the table-lookup step of a large index range scan (e.g. "WHERE state='Ca'", or "WHERE age is between 50 and 65") will be sequential-like IO (either really sequential, or close enough to sequential so that read-ahead still triggers, or falling into the same table-block as the previous row so requiring no IO at all) rather than random IO.

Also, since your relation is not monotonic but rather more like a parabola, the estimated correlation will undervalue the strength of the correlation in your case, as it only looks at linear correlations.