List<MyCustomObject__c> originalList = [Select Id, Name, Age From MyCustomObject__c Where Age = '20'];

List<MyCustomObject__c> cloneList = originalList.deepClone();

public static List<MyObject__c> getClones(String age)
{
    List<String> fields = new List<String>();
    FieldSet fieldsToClone = SObjectType.MyObject__c.fieldSets.FieldsToClone;
    for (FieldSetMember field : fieldsToClone.getFields()) {
        fields.add(field.getFieldPath());
    }
    String soql = 'SELECT ' + String.join(fields, ',') +
        ' FROM MyObject__c WHERE Age__c = '' + String.escapeSingleQuotes(age) + ''';
    return Database.query(soql).deepClone(...);
}