Understanding SQL Server statistics better
Very easy. Look for the result of: 

select datediff(day,'1/1/1950','12/31/2007');


It will give you your DISTINCT_RANGE_ROWS + 1
That means that SQL Server thinks that all your DOBs are approximately equally distributed across the range with average 236 items per day.
Query your data and see if you have any outlines like: missing day ranges or >1000 DOBs per day.
If you do, than that would be a question to the engine.