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, lastname
FROM friends
HAVING age >= AVG(age)

ERROR: Attribute friends.firstname must be GROUPed or used in an
aggregate function

This fails too:

SELECT firstname, lastname
FROM friends
WHERE age >= AVG(age)

ERROR: Aggregates not allowed in WHERE clause

This fails. I am stumped.

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