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:
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