Фильтрация двух связанных списков по нескольким параметрам

 class Student
    {
        public string name;
        public int age;
        public List<string> stList;
        public Student (string a, int b, List<string> c)
        {
            this.name = a;
            this.age = b;
            this.stList = c;
        }
    }
    class Section
    {
        public string sectName;
        public int opened;
        public List<string> sectList;
        public Section(string a, int b, List<string> c)
        {
            this.sectName = a;
            this.opened = b;
            this.sectList = c;
        }
    }
 
    class Program
    {
        static void Main(string[] args)
        {
            List<Student> students = new List<Student>
           {
               new Student ("Jack", 22, new List<string> { "Basketball" }),
               new Student ("Sam", 21, new List<string> { "Basketball" }),               
               new Student ("Alex", 19, new List<string> { "Boxing", }),               
               new Student ("Oscar", 20, new List<string> { "Boxing" }),               
               new Student ("Harry", 23, new List<string> { "Boxing" }),
               new Student ("William", 18, new List<string> { "Tennis" })
           };           
            List<Section> sections = new List<Section>
            {              
                new Section ("Boxing", 2001, new List<string> { "Alex", "Oskar", "Harry" }),
                new Section ("Tennis", 2007, new List<string> { "William" }),
                new Section ("Basketball", 1998, new List<string> { "Jack", "Sam" }),
            };
 
            var select2 = from sect in sections
                          from stud in students
                          where sect.opened > 2000
                          where stud.age < 21
                          select sect;
            foreach (var x in select2)
            {
                Console.WriteLine($"{x.sectName} {x.opened}");
            }
        }
    }

var query = from section in sections
            where section.opened > 2000
            join studentSection in
            (from student in students
                from section in student.stList
                select (section, student))
                on section.sectName equals studentSection.section into participants
            where participants.All(s => s.student.age < 21)
            select section;
 
foreach (var section in query)
    Console.WriteLine(section.sectName);

    class Student
    {
        public string Name { get; set; }
        public int Age { get; set; }
        public List<Section> Sections { get; set; }
        public Student(string Name, int Age, List<Section> Sections)
        {
            this.Name = Name;
            this.Age = Age;
            this.Sections = Sections;
        }
    }
    class Section
    {
        public string Name;
        public int Opened;
 
        public Section(string Name, int Opened)
        {
            this.Name = Name;
            this.Opened = Opened;
        }
    }

var select2 = sections.Where(section => 
    section.opened > 2000
    && section.sectList.All(
        name => students.First(student => student.name == name).age < 21
    )
);