class Data
{
    public string Name { get; set; }
    public string LastName { get; set; }
    public int Age { get; set; }

    public Data(string name, string lastname, int age)
    {
        Name = name;
        LastName = name;
        Age = age;
    }
}

static void Main(string[] args)
    {
        People[] people = new People[MaxPeople];
        people = ReadData();

        WriteKids(people);
        WriteAdults(people);
    }




static void WriteKids(Data[] people)
{
File.WriteAllText(@"Kids.csv", String.Empty);
using (StreamWriter writer = new StreamWriter(@"Kids.csv", true, Encoding.Default))
{
    for (int i = 0; i < Count; i++)
    {
        if (people[i].Age < 18)
        {
            writer.WriteLine("{0};{1};{2}", people[i].Name,              people[i].LastName, people[i].Age);
        }
    }
}


static void WriteAdults(Data[] people)
{
        File.WriteAllText(@"Adults.csv", String.Empty);
        using (StreamWriter writer = new StreamWriter(@"Adults.csv", true, Encoding.Default))
        {
            for (int i = 0; i < Count; i++)
            {
                if (people[i].Age => 18)
                {
                    writer.WriteLine("{0};{1};{2}", people[i].Name, people[i].LastName, people[i].Age);
                }
            }
        }
 }

private static void Main(string[] args)
{
    People[] people = new People[MaxPeople];
    people = ReadData();

    var kids = GetKids(people);
    var adults = GetAdults(people);
    WritePeople(kids, @"Kids.csv");
    WritePeople(adults, @"Adults.csv");
}

private static Data[] GetKids(Data[] people)
{
    return people.Where(x => x.Age < 18).ToArray();
}

private static Data[] GetAdults(Data[] people)
{
    return people.Where(x => x.Age >= 18).ToArray();
}

private static void WritePeople(Data[] people, string fileName)
{
    File.WriteAllText(fileName, String.Empty);
    using (StreamWriter writer = new StreamWriter(fileName, true, Encoding.Default))
    {
        foreach(var person in people)
        {
            writer.WriteLine("{0};{1};{2}", person.Name, person.LastName, person.Age);
        }
    }
}

static void writeKids(StreamWriter writer, string fileName){ }

for(int i = 0; i < count; i++){
     people[0].writeKids(...);
}