REQUIREMENTS FOR THE MULTI-ENGINE RATING

No age requirement.
No formal ground school.
No FAA regulated flight time is required, but you must have at least a private pilot certificate.

Most flight school and insurance companies have a minimum flight time for the mult-engine rating, usually somewhere between 5 to 10 hours of dual instruction.
 
Besides meeting the requirements found in FAR 61.63(c) - see below - you'll likely want to get your high performance and complex training (with the appropriate endorsements) in order to ACT as PIC. Most multiengine aircraft are both HP and complex.
 
Basically, you have to do training in the areas of operation found in FAR 61.107(b)(2) that cover the multiengine rating at the private level.
 
FAR 61.63(c) Additional class rating. Any person who applies for an additional class rating to be added on a pilot certificate:

1. Must have an endorsement in his or her logbook or training record from an authorized instructor and that endorsement must attest that the applicant has been found competent in the aeronautical knowledge areas appropriate to the pilot certificate for the aircraft class rating sought;
2. Must have an endorsement in his or her logbook or training record from an authorized instructor, and that endorsement must attest that the applicant has been found proficient in the areas of operation appropriate to the pilot certificate for the aircraft class rating sought;
3. Must pass the required practical test that is appropriate to the pilot certificate for the aircraft class rating sought;
4. Need not meet the specified training time requirements prescribed by this part that apply to the pilot certificate for the aircraft class rating sought unless the person holds a lighter-than-air category rating with a balloon class rating and is seeking an airship class rating and
5. Need not take an additional knowledge test, provided the applicant holds an airplane, rotorcraft, powered-lift, or airship rating at that pilot certificate level.