A fishing pole is in the Two-handed weapon item class, and can be wielded as a melee weapon, but is not a weapon type associated with a weapon skill. As a two-handed item, only one can be equipped at a time, and no item can be held in the off-hand.
You can purchase a basic fishing pole with any level character, and advanced poles become available as your fishing skill increases. Better poles grant a bonus to your fishing skill when equipped.
The basic Fishing Pole has no level requirement and no fishing skill requirement. You can purchase it, wield it, and use it as a melee weapon with no fishing training, but you cannot fish with it until you are a train in Apprentice Fishing. All characters will begin fishing using this item; even if you have access to the The Family and The Fishing Pole quest, the quest requires you to fish (for six Darkshore Groupers) before it awards you the Blump Family Fishing Pole.
Other fishing poles with no explicit level requirements have an implicit level requirement in the quest to obtain them.
The Arcanite Fishing Pole has no explicit level requirement, however competing in the Stranglethorn Fishing Extravaganza requires 130 fishing skill in order to fish, 225 fishing skill to fish with none getting away (it is a competition) (this can be done with Journeyman training, 150 fishing skill level, and lures), has level 31-40 mobs, and PvP if you are on a PvP server (turn off your flag for this event if you are not) (these hazards can be handled at a low level with a group of higher level friends as guards). You must be level 35 to train in Artisan level fishing in order to reach fishing skill level 300 and equip the Arcanite Fishing Rod.
Also note that only one Stranglethorn Fishing Extravaganza prize is given out each week - this is effectively a seven day cool down per attempt to obtain one.
The Fishing Line items produce a permanent fishing skill enchantment on a fishing pole:
How to Obtain
Fishing Lines — permanent enchantment on fishing pole, only one at a time
The fishing skill requirement to use the fishing line is the fishing skill required to expend the enchantment charge and apply it to a fishing pole. The requirements of the fishing pole are not changed, nor does an unbound pole become bound.
You must be level 70 to participate in the daily fishing quests, so the Spun Truesilver Fishing Line has an implicit level requirement.
The High Test Eternium Fishing Line has no explicit level requirement, and the turn-in to obtain it is not competitive (not in the sense that only one is awarded). A low level character fishing in Stranglethorn Vale's coastal waters will require assistance.
Fishing Pole With Fishing Line
Because the Fishing Line items produce a permanent fishing skill enchantment on a fishing pole (consuming the fishing line), they can be viewed as an investment in your fishing pole:
Note that the poles that can be traded can still be traded after line is applied.
The fishing lines can not be traded, since they are BoP items, but it is clear that they have a high value. It would be a waste to apply one to a fishing rod that had less than a significant bonus to fishing skill.