public IList<PersonAgeGroup> GroupPersonsByAge(List<Person> persons)
        {
            //throw new NotImplementedException();
            var twentiers = persons.GroupBy(p => p.Age == 20);
            var thirtiers = persons.GroupBy(p => p.Age == 30);
            List<PersonAgeGroup> personAgeGroups = new List<PersonAgeGroup>;
            personAgeGroups[0].Age = 20;
            personAgeGroups[0].Persons = twentiers;
            personAgeGroups[1].Age = 30;
            personAgeGroups[1].Persons = thirtiers;

            return personAgeGroups;
        }