Get latest timestamp from column in given table dynamically
Probably simplest with regclass as input and an OUT parameter:
CREATE OR REPLACE FUNCTION f_last_update(_tbl regclass, OUT _last_update timestamp)
  LANGUAGE plpgsql AS
$func$
BEGIN
   EXECUTE 'SELECT max(updated_at) FROM ' || _tbl
   INTO _last_update;
END
$func$;

Call:
SELECT f_last_update('mytable');

Optionally schema-qualified and double-quoted where required:
SELECT f_last_update('&quot;My_odd_ScHeMa&quot;.&quot;Unwise table name&quot;');

Safe against SQL injection because regclass is automatically quoted and schema-qualified properly when converted to text.
See:

Table name as a PostgreSQL function parameter
Find tables with columns with empty and NULL values in Postgresql
Check whether empty strings are present in character-type columns