Can a minor be the partner in a business?
A business partnership is defined as:
An association of two or more persons engaged in a business enterprise in which the profits and losses are shared proportionally. The legal definition of a partnership is generally stated as "an association of two or more persons to carry on as co-owners a business for profit" (Revised Uniform Partnership Act § 101 )
Typically such arrangements are walked into under written contract denoting roles and responsibilities of each partner, as well as the financial breakdown, "voting power," or percent of ownership in the company of each partner. Assuming that you are defining "minor" to mean persons under the age of 18 and in America, certainly, if you look for real life examples, you will see that it is possible for minors to be partners in and even owners of businesses.
It doesn't happen often but it certainly happens. It would seem that most of the young business "partners," or "presidents" obtained their positions by founding the company in the first place. It is also likely more common for a minor to be accepted as a partner into a "family" business. There would certainly be more legal issues to deal with while the person is under the age of 18, but the simple answer to your question, "Can it be done?" is, yes, but contractural agreements may be cancelled, or disaffirmed, by the minor at any time, with a few exceptions like entertainment contracts.