Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_60898 ( "Romanised name" text, "Chinese name" text, "age at appointment" real, "Foreign nationality" text, "Portfolio attachment" text, "Govt salary" text )

### Question ###
What is the average age at appointment of those attached to security?

### Accurate SQL ###

SELECT AVG("age at appointment") FROM table_60898 WHERE "Portfolio attachment" = 'security'