I startet fishing from the start with my first character. It helped me with the cooking skill and got some fish for the char from my friend who could use them for alchemy. Leveling as an Alliance char you have lot of time to wait for ships, so there was always an opportunity to fish, while waiting.
All my characters (Alli and Horde) startet fishing with the first levels and have their skill maxed before they reach max. level. I always did the fishing quests at Shattrath and now at Dalaran. Gives you nice Items, some Gold and even reputation.
I'm trying to make the achievments for fishing, but without a hurry. Since I play 9 Chars between level 70 and 80, there is so much to do.
I hope, that someday I catch the Turtle and maybe another Dr. Pinchy, who gives me the Pet. (Got two without it).