most pokemon who "die" come back as ghost types however
There's no evidence of that. Only Marowak in red/green/blue comes back as a ghost. But not as a pre-existing ghost pokemon.
Several ghost types are spirits infesting non-pokemon things, like Yamask or Banette.
They can also be bred and hatched.
And none of this is relevant to the fact that pikachu survived for over 50 years without aging a bit.
