Try this:
create trigger STEP5
after insert on donates2
for each row
declare
donage number;
recage number;
begin
SELECT b.AGE
INTO DONAGE
FROM PERSONS b
WHERE b.PNAME = :NEW.DONOR;
SELECT c.AGE
INTO RECAGE
FROM PERSONS c
WHERE c.PNAME = :NEW.RECEIVER;
if donage = recage then
dbms_output.put_line(:NEW.DONOR|| ' and ' || :NEW.RECEIVER || ' ARE peers');
end if;
end STEPS;
Best of luck.
Share and enjoy.