The POP3 protocol is very simple and doesn't define any sorting. This is what RFC 1939 says:
After the POP3 server has opened the maildrop, it assigns a message-
number to each message, and notes the size of each message in octets.
The first message in the maildrop is assigned a message-number of
"1", the second is assigned "2", and so on, so that the nth message
in a maildrop is assigned a message-number of "n". In POP3 commands
and responses, all message-numbers and message sizes are expressed in
base-10 (i.e., decimal).
Very often the POP3 server assign numbers in ascending order as the message comes. So the oldest message has number 1. However, it is not guaranteed (you should check server's documentation if it is guaranteed).