I need help on how to calculate age of a child in months using a masked text box and the answer should be shown in textbox (in months)

private void AGE_CALC_TextChanged(object sender, EventArgs e)
{
	try
	{
		if (AGE_FINAL.Text.Length >= 1)
		{
			if (Convert.ToDateTime(DATEBIRTH.Text).Year == 1908)
			{
				double age_days = (System.DateTime.Now -
                                  Convert.ToDateTime(DATEBIRTH.Text)).TotalDays;

				//(EndDate - StartDate).TotalDays
				//MessageBox.Show(age_days.ToString());
				if (age_days > 183 && age_days <= 1825)
				{
					CHILDAGE.Text = "1";
					CHILDAGE.Enabled = false;
				}
				else
				{
					CHILDAGE.Text = "2";
					CHILDAGE.Enabled = false;
				}
			}
			else if (Convert.ToDateTime(DATEBIRTH.Text).Year != 1908)
			{
				double age_days = (System.DateTime.Now -
                                  Convert.ToDateTime(DATEBIRTH.Text)).TotalDays;

				//(EndDate - StartDate).TotalDays
				//MessageBox.Show(age_days.ToString());
				if (age_days > 183 && age_days <= 1825)
				{
				   // MessageBox.Show(age_days.ToString());
					CHILDAGE.Text = "1";
					CHILDAGE.Enabled = false;
					SEX.Focus();
				}
				else
				//else if (age_days <= 183 && age_days > 1825)
				{
					// MessageBox.Show(age_days.ToString());
					CHILDAGE.Text = "2";
					CHILDAGE.Enabled = false;
					SEX.Focus();
				}
			}
		}
	}

	catch
	{
		MessageBox.Show("Please put in a date of birth if known", "ZTDT");
	}
}