The UPSC CAPF Assistant Commandant exam has a fixed minimum and maximum age on the cutoff date given in the notification.
Age relaxation applies to reserved categories — SC, ST, OBC and ex-servicemen — as per government rules.
Your age is counted as on the 1st of August of the exam year, unless the notification says otherwise.
Check the official UPSC notification for the exact limits for your category.
Never guess the cutoff date — a wrong date can make your application invalid.