public class User
{
    public int UserID { get; set; }
    public string FirstName {get; set;}
    public string LastName {get;set;}
    public bool? Active {get;set;}
    public int? Age { get; set; }
    public string Sex { get; set; }

}

// Initialize just two fields and leave the other to their defaults
// (null for both strings and nullable ints)
User u = new User();
u.UserID = 1;
u.FirstName = "Steve";
bool ok = UpdateUser(u);
if(ok) ......


public UpdateUser(User info)
{
    using(SqlConnection cnn = new SqlConnection(@"Data Source=(LOCAL);
                                                Initial Catalog=TestDB;
                                                Integrated Security=True;"))
    {
        cnn.Open();

        // Prepare the parameters to pass to the Dapper Execute
        var pms = new
        {
           UserID = info.UserID
           FirstName = info.FirstName,
           LastName = info.LastName,  // <- from here all is null
           Active = info.Active,
           Age = info.Age,
           Sex = info.Sex
        };

        int rows = cnn.Execute(@"UPDATE [UserTable]
                                 SET FirstName= @FirstName,
                                 LastName = @LastName,
                                 Active = @Active,
                                 Age = @Age,
                                 Sex = @Sex
                                 WHERE UserID = @UserID",
                                 pms);
         return rows != 0;
    }
}