Mo. Rev. Stat. § 442.040: Person under eighteen years of age may join in conveyance with adult spouse.

When any person under eighteen years of age is married to an adult who has or claims any interest in real estate and wishes to convey, encumber, lease or otherwise dispose or affect the same, the minor shall be deemed of age for the purpose of joining with his adult spouse in the execution of any instrument affecting the spouse's real estate.