user: Question: What is the average age at appointment of those attached to Security?
Columns: ['Romanised name', 'Chinese name', 'age at appointment', 'Foreign nationality', 'Portfolio attachment', 'Govt salary']
Types: ['text', 'text', 'real', 'text', 'text', 'text']
Sample row: ['Chan Chi-yuen, Paul', '陳智遠', 28.0, 'N/A', 'Food and Health', 'HK$ 134,150']
SQL:

assistant: SELECT AVG("age at appointment") FROM "Table" WHERE "Portfolio attachment" = 'Security';