If you want to count the number of rows where the age is null, you probably want this:
select count(*) from dbo.prim
where age is null
Count(*) means count the number of rows. Count(<column name>) means count the number of rows where that column has a non-null value.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability