In Pokémon Sapphire, Pikachu would have Growl and Thundershock upon capture and then it learns Tail Whip at Level 6, Thunder Wave at Level 8, Quick Attack at Level 11, Double Team at Level 15, Slam at Level 20, Thunderbolt at Level 26, Agility at Level 33, Thunder at Level 41 and Light Screen at Level 50.
Electrike/Manetric learns Thunder at level 36. Plusle/Minun learn Thunder at level 37. Then there's always Pokemon from the previous generations that can learn it as well. Like Ampharos, Pikachu, Elekid, etc.
Near 21 level. I have Pikachu of 21 level and it has recently learnt slam.
Level 31
Pikachu learns Thunderbolt at lvl 26 in Pokemon Yellow.
Pikachu learns Electro Ball at Level 18.
It can learn it via the TM but not by level.
No.
lvl 41
He Doesnt.
Yes, it learns Thunderbolt at level 26.
lvl 59
In Pokemon Ruby, Sapphire, and Emerald, Nincada can learn Dig at level 45. It can learn Dig at a earlier level by using TM 28.