Alvin is older. In one of the episode "Grounded Chipmunk" Alvin mentioned that the only other time he spent alone was five minutes before Simon was born. I think in other episodes it was said that Alvin was older (but not by much, lol).
Hope that helps...
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They're roughly the same age since they are triplets. But officially like one of the other posters said, it was stated in the episode "Grounded Chipmunk" that Alvin was born first, then Simon five minutes later and I assume Theodore sometime after that.
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Their birth order was gradually revealed throughout the series.
Chipmunks, oldest to youngest:
Alvin
Simon
Theodore
Chipettes, oldest to youngest:
Brittany
Jeanette
Eleanor
Alvin made a comment one episode where he was older than Simon by 5 minutes. The Chipmunks Reunion reveals that all 3 share the same birthday, so they are triplets. Another episode Brittany calls Jeanette her little sister, but this could be the same case as with the Chipmunks. However, the new CGI version shows that Jeanette has a different birthday than her sisters.