See also https://www.law.cornell.edu/uscode/text/42/1396p
Section 1396p(c)(2)(B)(iv)
Basically it is this:
1) The trust has to be irrevocable
2) The trust has to be established for the sole benefit of a disabled beneficiary (as defined in the Act).
3) The beneficiary has to be under 65 when the trust is established.
4) The beneficiary must have a disability that is included in the SSA.
