practice/ClassLib000/TestModel/Class1.cs

using System;

namespace ClassLib000.TestModel
{
    public class Car
    {
        private const int MaxAge = 20;
        public string PlateNumber;
        public string Color;
        public int AgeYears = default(int);
        private bool AgedEnough = false;

        public void PassYear()
        {
            if (!AgedEnough)
            {
                this.AgeYears++;
            }
            if (AgeYears > MaxAge)
            {
                AgedEnough = true;
            }
        }

    }
}