Using aggregate in HAVING

How would I get all friends greater than the average age?
CREATE TABLE friends (     firstname CHAR(15),     lastname CHAR(20),     age INTEGER)
SELECT firstname, lastnameFROM friendsHAVING age >= AVG(age)ERROR:  Attribute friends.firstname must be GROUPed or used
inanaggregate function
 

This fails too:
SELECT firstname, lastnameFROM friendsWHERE age >= AVG(age)ERROR:  Aggregates not allowed in WHERE clause

This fails.  I am stumped.

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