Section 21. An action for the recovery of land shall be commenced, or an entry made thereon, only within twenty years after the right of action or of entry first accrued, or within twenty years after the demandant or the person making the entry, or those under whom they claim, have been seized or possessed of the premises; provided, however, that this section shall not bar an action by or on behalf of a nonprofit land conservation corporation or trust for the recovery of land or interests in land held for conservation, parks, recreation, water protection or wildlife protection purposes.