Certified Flight Instructor (CFI) Rating Requirements:

Minimum 18 Years Old to Take Either the Commercial (or Certified Flight Instructor) Practical Flight Test
Hold a Commercial Pilot Helicopter Certificate with a Minimum of 150 Hours of Flight Time
Must Read, Write, Understand, and Speak English
Hold a Current FAA Medical Certificate
Pass FAA Written "Fundamentals of Instruction" and Helicopter Flight Instructor Written "Knowledge Tests"
Pass an FAA Oral and Practical Flight Test
Before you can earn your Helicopter Certified Flight Instructor (CFI) Rating, you must already hold a Helicopter Commercial Pilot Rating. Unlike in airplanes, you are NOT required to hold an Instrument Rating in either helicopters OR airplanes to become a Helicopter CFI. There are no additional flight experience requirements in the helicopter beyond preparing with a qualified Instructor Pilot (CFI) for the Oral and Practical Flight Test (or "Check Ride") with the FAA examiner. 

You will be expected to meet the standards in the FAA Practical Test Standard (PTS) for the helicopter Flight Instructor Rating. You will be expected to demonstrate all the Commercial PTS flight maneuvers from the "non-PIC" (Instructor's) seat in the aircraft as you explain those maneuvers as you would to a student pilot. You are also required to demonstrate a "touchdown" autorotation to the ground ("touchdowns" are NOT required for the Private or Commercial Rating). 
You will also be required to pass a rigorous Oral Examination with an FAA Inspector on the subject matters covered in the FAA's Certified Flight Instructor Practial Test Standard (PTS) publication. 

You are also required to pass two written FAA "Knowledge Tests." One covers the "Fundamentals of Instruction" and covers how people learn and how they should be taught. The other is a test on general aviation information that any Certified Flight Instructor should know. 

If you already hold an Airplane CFI Rating (or higher), you DO NOT need to take the "Fundamentals of Instruction" written test again, but you WILL need to take a very short Helicopter CFI Written "Knowledge Test", which consists of 20 general helicopter questions. 

Note: You can begin your flight training in the helicopter TODAY! There are NO prior requirements before you begin dual instruction in the helicopter. You can begin at any age, but you must be 16 years old to solo, 17 to earn your Private Pilot Certificate, and 18 to qualify for the Commercial Certificate and Flight Instructor Rating. (See our pages on Private Pilot and Commercial Pilot Requirements) 

FAA Requirements References: FAR 61.183; 61.185; 61.187

Hold an FAA Commercial Pilot Certificate
Hold a Current Third Class Medical (or Higher)
Log 3 Hours in Dual Instruction (Minimum) in the Previous 60 Days Prior to the Practical Flight Test (Checkride)
Have Passed The "Fundamentals of Instruction" FAA Written "Knowledge Test" With the Previous 24 Months
Have Passed The FAA Written Helicopter Flight Instructor (CFI) "Knowledge Test" With the Previous 24 Months
Pass the FAA Flight Instructor Practical Test (Checkride) Oral and Flight PortionNote: For the CFI "Add-On" Rating, you are NOT required to take the "Fundamentals of Instruction (FOI)" test again. You ARE, however, required to take a shorter, 20-question "CFI Helicopter Add-On" FAA Knowledge Test. Call for Details.