China*
Mongolia (I can't make up my mind whether he should be older or younger than Japan because in a chibi sketch that involves China, Mongolia and baby!Japan has him around the same physical age as China)
Japan*
Taiwan (I'm not really sure why Taiwan's here as she could be younger)
Thailand (If you beleive he was also Siam)
Vietnam (I'm sure she'd be younger than Thailand)
Tibet (Tibet has a VERY fuzzy timeline, he could be as old as China for all I know)
South Korea (I put Yong Soo here based on his independence as SOUTH KOREA not as THE KINGDOM OF KOREA)
*(This one's confusing as according to Hetalia canon China's at least in his teens while Japan is a child, even though apparently Japan was founded in 660BCE while China was founded in 221BCE, so either China was more developed than Japan hence why China was older physically than Japan at the time or something)
By Independence/Foundation
Japan (660BCE)
China (221BCE)
Mongolia (1206)
Thailand (1238)
South Korea (1945, August 15th)
Vietnam (1945, September 2nd)
Source:
link
(Tibet and Taiwan aren't on the list)
Overall it's quite hard to say because of Asia's very fuzzy timeline. Also we don't know whether these guys can exist before their independence. A good example is North and South Korea. Did they exist before the penisula was split into two countries as the Koreans have been around for very long or was their was a different person representing the Kingdom of Korea?