Real property upon which the county holds a tax certificate shall continue liable to taxation, but when a tax deed shall be issued to the county such property shall thereafter be exempt from taxation until the same is sold by the county. The county clerk shall annually, before February 1, furnish to the assessors of each town a list of the lands in such town exempt under this section. Nothing in this section shall be so construed as to apply to lands owned by minors or persons adjudged mentally incompetent.