Age is counted on a fixed cutoff date given in the notification, not on the day you apply. If a post asks for 18 to 27 years as on 1 August, you must have turned 18 but not yet turned 27 on that date: born on or after 2 August of the year 27 years earlier and on or before 1 August of the year 18 years earlier. The notification always prints this date-of-birth window.
How does category age relaxation work?
Relaxation is added to the upper age limit only. For most central government posts it is 3 years for OBC (Non-Creamy Layer), 5 years for SC and ST, and 10 years for PwBD (13 for PwBD OBC, 15 for PwBD SC/ST). Check your exam notification, as some exams differ.