Well, there are a few reasons to start on an older server.
1) Servers 1-10 have a different set-up with five realms and five sets of cities to fight over. This experience does not exist on newer servers.
2) Lots of experienced players to learn from.
3) Relative safety. You are so much lower in level, other players cannot attack you in the wild. This is great for PvE players.

Downside: well, guild wars and sailing are a real pain because you ARE so much lower level. Anything concerning PvP will be a problem, as you are very weak in comparison.