A spendthrift trust may not continue for a period longer than that allowed under NRS 111.103 to 111.1039, inclusive. The free alienation of the legal estate by the trustee may not be suspended for a period exceeding the limit prescribed in any constitutional or statutory prohibition against perpetuities existing in this State or in the state where the lands affected by the trust are situate, but a contingent remainder in fee may be created on a prior remainder in fee, to take effect if the persons to whom the first remainder is limited die under the age of 21 years, or upon any other contingency by which the estate of those persons may be determined before they attain that age.