declare
birthdate date :='23-jan-1979';
begin
dbms_output.put_line('Your age is 
'||(to_char(sysdate,'yyyy')-to_char(birthdate,'yyyy'))||' years 
'||(to_char(sysdate,'MM')-to_char(birthdate,'MM'))||' months
'||(to_char(sysdate,'DD')-to_char(birthdate,'DD'))||' days');
end;
/

Your age is 
29 years 
0 months
1 days

SQL> declare
  2  birthdate date :='23-jan-1979';
  3  begin
  4  dbms_output.put_line('Your age is 
  5  '||(to_char(sysdate,'yyyy')-to_char(birthdate,'yyyy'))||' years 
  6  '||(to_char(sysdate,'MM')-to_char(birthdate,'MM'))||' months
  7  '||(to_char(sysdate,'DD')-to_char(birthdate,'DD'))||' days');
  8  end;
  9  /
'||(to_char(sysdate,'MM')-to_char(birthdate,'MM'))||' months
                 *
ERROR at line 6:
ORA-01858: a non-numeric character was found where a numeric was expected
ORA-06512: at line 2

SQL> declare
  2  birthdate date := to_date('13/11/1956','DD/MM/YYYY');
  3  begin
  4  dbms_output.put_line('Your age is 
  5  '||(to_char(sysdate,'yyyy')-to_char(birthdate,'yyyy'))||' years 
  6  '||(to_char(sysdate,'MM')-to_char(birthdate,'MM'))||' months
  7  '||(to_char(sysdate,'DD')-to_char(birthdate,'DD'))||' days');
  8  end;
  9  /
Your age is
52 years
-10 months
11 days

PL/SQL procedure successfully completed.

SQL> declare
  2  birthdate date :='31-dec-1979';
  3  begin
  4  dbms_output.put_line('Your age is
  5  '||(to_char(sysdate,'yyyy')-to_char(birthdate,'yyyy'))||' years
  6  '||(to_char(sysdate,'MM')-to_char(birthdate,'MM'))||' months
  7  '||(to_char(sysdate,'DD')-to_char(birthdate,'DD'))||' days');
  8  end;
  9  /
Your age is
29 years
-11 months
-7 days

PL/SQL procedure successfully completed.

SQL>

declare
birthdate date :='30-dec-1979';
dd number;
mm number;
yyyy number;
begin
select (to_char(sysdate,'yyyy')-to_char(birthdate,'yyyy')) into yyyy from  dual;
select (to_char(sysdate,'MM')-to_char(birthdate,'MM')) into mm from dual;
select (to_char(sysdate,'DD')-to_char(birthdate,'DD')) into dd from dual;
if dd<0
then dd :=dd+30;
mm :=mm-1;
end if;
if mm<0
then mm :=mm+12;
yyyy :=yyyy-1;
end if;
dbms_output.put_line('Your age is
'||yyyy||' years
'||MM||' months
'||DD||' days');
end;

Your age is
28 years
0 months
25 days

PL/SQL procedure successfully completed.

SQL> ed
Wrote file afiedt.buf

  1  declare
  2  birthdate date :='30-dec-1979';
  3  dd number :=(to_char(sysdate,'DD')-to_char(birthdate,'DD'));
  4  mm number :=(to_char(sysdate,'MM')-to_char(birthdate,'MM'));
  5  yyyy number  :=(to_char(sysdate,'yyyy')-to_char(birthdate,'yyyy'));
  6  begin
  7  if dd<0
  8  then dd :=dd+30;
  9  mm :=mm-1;
 10  end if;
 11  if mm<0
 12  then mm :=mm+12;
 13  yyyy :=yyyy-1;
 14  end if;
 15  dbms_output.put_line('Your age is
 16  '||yyyy||' years
 17  '||MM||' months
 18  '||DD||' days');
 19* end;
SQL> /
Your age is
28 years
0 months
25 days

PL/SQL procedure successfully completed.