OLDEST NATIONAL FLAGS BY THE DATE OF ADOPTION:

1. 🇩🇰 Denmark - 1625
2. 🇳🇱 Netherlands - 1660
3. 🇬🇧 United Kingdom - 1801
4. 🇦🇷 Argentina - 1812
5. 🇨🇱 Chile - 1817
6. 🇵🇪 Peru - 1825
7. 🇺🇾 Uruguay - 1828
8. 🇫🇷 France - 1830
9. 🇹🇳 Tunisia - 1831
10. 🇧🇪 Belgium - 1831
11. 🇵🇾 Paraguay - 1842
12. 🇹🇷 Türkiye - 1844
13. 🇱🇷 Liberia - 1847
14. 🇧🇴 Bolivia - 1851
15. 🇪🇨 Ecuador - 1860
16. 🇨🇴 Colombia - 1861
17. 🇸🇲 San Marino - 1862
18. 🇭🇳 Honduras - 1866
19. 🇯🇵 Japan - 1870
20. 🇬🇹 Guatemala - 1871
21. 🇹🇴 Tonga - 1875
22. 🇲🇨 Monaco - 1881
23. 🇰🇷 South Korea - 1883
24. 🇨🇭 Switzerland - 1889
25. 🇵🇭 Philippines - 1898
26. 🇳🇴 Norway - 1899
27. 🇦🇺 Australia - 1901
28. 🇨🇺 Cuba - 1902
29. 🇳🇿 New Zealand - 1902
30. 🇸🇪 Sweden - 1906
Note: adoption of the current Flag design

Source: Wikipedia, Guinness World Records, Others