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

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Christine Penner
Ingenious Software
250-352-9495
chris(at)fp2(dot)ca