Some very old, some sort of old, some probably built within the last century. The really old ones are concentrated in certain place, Ayuttaya and Sukhothai. These are older cities in the middle of the country, both served as capital cities back when Bangkok was just a marshy place down river.
Even though I’m not a Buddhist, I dug visiting these really ancient piles of brick, especially the ones with vegetation growing out of them. They’re every bit as impressive as Aztec or Mayan ruins, and the ruined Jesuit missions of Paraguay and Argentina.
As anyone who has toured the cathedrals of Europe or Latin America knows, they all start to look the same after a while. Same with these Thai temples, at least to this Catholic Midwesterner. But Thai Buddhists probably don’t think so, and this is definitely a Buddhist country. Some people try to say the United States is a Christian country. It isn’t. But Thailand is definitely a Buddhist country. The King is the head of the church and the Buddhist church is enshrined in the Constitution.