adding years to a date field

I have to add a number of years to a date field. The years come from another field in the table.

I know I can do this:
select (ii_purchased + interval  '3 year') as date from inventory_item;

But I need to replace the 3 in 3 years with another field from the same table ii_expected_life but I can't seem to do that without getting syntax errors

-- 
Christine Penner
Ingenious Software
250-352-9495
chris@fp2.ca