назва_змінної.infants.ToString();

private void Form1_Load(object sender, EventArgs e)
        {

            TextBox1.Text = Age_Organizer.infants_r + "";

        }

class Program
{
    static void Main(string[] args)
    {
        //Application.EnableVisualStyles();
        //Application.SetCompatibleTextRenderingDefault(false);
        //Application.Run(new Form1());
        // The commented lines are relevant only for the WinForms version. Perhaps it was necessary to add my code to the form differently?
        int count = 0, gender = 0, sity;
        Regex regex = new Regex(@"\d{2}\D\d{2}\D\d{4}");
        Regex regex_2 = new Regex(@"female");
        string sourceDirectory = @"F:\Direction"; 
        foreach (var txtFiles in Directory.EnumerateFiles(sourceDirectory, "*.html"))
        {
            Some_Parser_Work(); // Тут починає свою работу парсер. Насправді немає окремого методу для цього, парсер описаний в головному методі.
            Age_Organizer.AgeOrg(years, months, days, gender, sity); // years, months, days - локальні зміннні
            break;    
        }
            count = 0;
        Age_Organizer.AgeShow(); // Not used in WinForms version.
        Console.ReadKey(); // Not used in WinForms version.
    }
}

public class Age_Organizer 
{
    public static int man_r = 0, woman_r = 0, sity_1 = 0;
    public static int age0_5_r = 0, age0_5_r_f = 0;
    public static int infants_r = 0, kids_r = 0, teenagers_r = 0, tenage_workers_r = 0, adult_workers_r = 0, adult_pensioners_p = 0;
    public static int infants_r_f = 0, kids_r_f = 0, teenagers_r_f = 0, tenage_workers_r_f = 0, adult_workers_r_f = 0, adult_pensioners_p_f = 0;
    public static int m0_r = 0, m1_r = 0, m2_r = 0, m3_r = 0;
    public static int m0_r_f = 0, m1_r_f = 0, m2_r_f = 0, m3_r_f = 0;
    // Насправді полів набагато більше, але суті це не змінює.
    public static void AgeOrg(int years, int months, int days, int gender, int sity)
    {
        if (sity == 1)
        {
            sity_1++;
            if (gender == 1)
            {
                woman_r++;
                if (years < 1)
                {
                    infants_r_f++;
                    age0_5_r_f++;
                    if (months == 0)
                        m0_r_f++;
                    if (months == 1)
                        m1_r_f++;
                    if (months == 2)
                        m2_r_f++;
                    if (months == 3)
                        m3_r_f++;
                }
                if (gender == 2)
                {
                    man_r++;
                    if (years < 1)
                    {
                        infants_r++;
                        age0_5_r++;
                        if (months == 0)
                            m0_r++;
                        if (months == 1)
                            m1_r++;
                        if (months == 2)
                            m2_r++;
                        if (months == 3)
                            m3_r++;
                    }

                }
            }
        }
    }
    public static void AgeShow() // This method is described exclusively for the console version.
    {
        Console.WriteLine("Statistics for City 1 (" + (woman_r + man_r) + " served inhabitants - " ):\n");
        Console.WriteLine("Age groups: \t\t0-5");
        Console.WriteLine("Female gender:  \t{0}", age0_5_r_f);
        Console.WriteLine("Male gender: \t{0}", age0_5_r);
        Console.WriteLine("\nChildren:\tinfants (0 years)   Kids (0-14)   Teenagers (15-17)   tenage_workers (16-17)   Total ()");
        Console.WriteLine("Female gender::  \t\t{0}\t\t{1}\t\t{2}\t\t\t{3}\t\t\t{4}", (infants_r_f + kids_r_f), teenagers_r_f, tenage_workers_r_f, (infants_r_f + kids_r_f + teenagers_r_f));
        Console.WriteLine("Male gender: \t\t{0}\t\t{1}\t\t{2}\t\t\t{3}\t\t\t{4}\n", (infants_r + kids_r), teenagers_r, tenage_workers_r, (infants_r + kids_r + teenagers_r));
    }
}

if (gender == 1)
{
    A();
}
if (gender == 2)
{
    B();
}