timespan question: using value from row in select

Hi folks,

I'm trying to write a SELECT statement that uses a min_age column
(integer expressing a number of years) as part of a timespan:

select * from users U where
age('now', '01-01-1970') >= timespan('U.min_age years'::timespan);
^^^^^^^^^^^^^^^^^^^^^^^^^^^

ERROR: parser: parse error at or near "error"

Is there some way to do this? I want to convert an int expressing years
into a timespan, within a select statement.

Thanks for any insights on how to do this.

--Michael