public class Person
    {
        public string Name { get; set; }
        public int Age { get; set; }
 
        /* other attribe */
    }

  public class Dept
    {
        public List<Person> Employee { get; set; }

        public List<Dept> DivDept { get; set; }
    }

static void Main(string[] args)
        {
            List<Person> result = new List<Person>();
            List<Dept> all = new List<Dept>();

            all.ForEach(dept => GetAgeOver(dept, result));
        }


        static void GetAgeOver(Dept dept, List<Person> result)
        {
            if (dept.Employee != null)
            {
                dept.Employee.ForEach(em =>
                {
                    if (em.Age > 25)
                    {
                        result.Add(em);
                    }
                });
            }

            dept.DivDept.ForEach(subDept => GetAgeOver(subDept, result));
        }