A clause in any policy of life insurance providing that such policy shall be incontestable after a specified period shall preclude only a contest of the validity of the policy, and shall not preclude the assertion at any time of defenses based upon provisions in the policy which exclude or restrict coverage, whether or not such restrictions or exclusions are excepted in such clause; nor shall it be construed to preclude adjustment at any time of the amount payable or benefits accruing under the policy for misstatement of age, whether or not such age adjustment provision is excepted in such clause.