Finding the average length of a column as per a particular case in MySQL

Finding the average length of a column as per a particular case in MySQL

I have a table of Users and another table of the Messages they've sent and
I'm trying to find the average length of a message and the average length
of a message if the user is retired. If I try this:
SELECT u.id,
AVG(LENGTH(m.body)) AS avg_msg_length,
AVG(LENGTH(CASE m.is_retired WHEN true THEN m.body ELSE NULL END)) AS
avg_msg_length_retired
FROM Users u LEFT OUTER JOIN Messages m
ON u.id = m.sender_id
GROUP BY u.id;
In the result I get the average length fine but the average length for
retired people is just a column of NULLs.
If I try this:
SELECT u.id,
AVG(LENGTH(m.body)) AS avg_msg_length,
AVG(LENGTH(CASE m.is_retired WHEN "true" THEN m.body ELSE NULL END)) AS
avg_msg_length_retired
FROM Users u LEFT OUTER JOIN Messages m
ON u.id = m.sender_id
GROUP BY u.id;
I get ERROR 1054 (42S22): Unknown column '"true"' in 'field list'. I find
this extremely strange since the is_retired field is a varchar.
Am I using the CASE WHEN incorrectly inside the LENGTH? I tried the second
form of CASE WHEN too as:
AVG(LENGTH(CASE WHEN m.is_retired=true THEN m.body ELSE NULL END)) AS
avg_msg_length_retired
and,
AVG(LENGTH(CASE WHEN m.is_retired="true" THEN m.body ELSE NULL END)) AS
avg_msg_length_retired
And get the same results as above.