person.ts

class Person {
  private _age: number;
  
  public set age(age) {
    if (age < 0 || age > 200) {
      throw new Error('Invalid arguement age');
    }
    
    this._age = age;
  }
  
  public get age() {
    return this._age;
  }
}

const person: Person = new Person();

// You dont have to use the method here, just assign the value
person.age = -10;