Hi,
In order to  answer your question I would like to put all 
three possible answers as follows:-

1.If You want to find out the age of employee as on the 
date of Joining (Field name EDOJ) then Query will be

select *,datediff(year,EDOB,EDOJ) as Eage from emp

2. If you want to find out the age of employees as on 
current date, query will be as follows:-
 
select *,datediff(year,EDOB,getdate()) from emp

3. If you want to find out age as on specific date stored 
and taken from other variable then query will be :-

You need to declare a variable as datetime then need to 
tore its value to it.

declare @dtb datetime
set @dtb='1990/01/10'
select *,datediff(year,EDOB,@dtb) from emp
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For any clarification Please feel free to contact me.
Rajesh
9342284359

Thanks Rajesh K. Gupta.

select trunc(months_between(Sysdate,EDOB)/12)Age from emp;

The answer submitted by Santhi K is wrong, Can shanti test 
this query in sql server, because i ran it it does not 
accept. there is no such months_between function

Answer i submitted is for Oracle not for SQL server.
Sorry!
Plz always mention the RDBMS which you are using ,in the 
post.
Bcoz,This Oracle query may not work in other RDBMS.

FOR SQL 2005:

select datediff (day, birthdate, getdate())/365 from emp