John Goodman, the Florida millionaire,  has notoriously adopted his girlfriend, who is only six years his junior. 

Trial and heirs blog has a very interesting analysis of the matter here. As the authors explain it, a primary reason for the adoption is that Goodman's irrevocable trust for his children needed tending to. 

It seems the trustee of the trust, in Goodman's view, was not managing it well on behalf of his teenage children. And since the trust was irrevocable, Goodman himself could not do anything to change the trustee. 

Enter his girlfriend. Goodman's plan appears to have been to make her his legal "child," thus giving her the right to more closely monitor the behavior of the trustee from the standpoint of a beneficiary of the trust. 

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Tanner Pittman, LLC is a LaGrange, Georgia, and metro Atlanta law firm that advises clients on estate planning matters, including irrevocable trusts