Well, we would have to distinguish between naturally appearing ghosts/spirits, which would be some kind of monster species, and ghosts, that appear when a person dies.
In the first case, as they are their own species different from humans, their concept and age of maturity may be different from ours. Maybe these ghosts become mature at the age of 8, so it's not too far fetched too think about a ghost loli harem in my opinion, although, further research has to be done.
In the second case we are talking about entities, that have become ghosts after their death. So we should still regard them as humans, which would at first let it seem like, that ghost lolis are not legal. BUT!
The pressing question is: Do ghosts maybe mature?
If that were the case you maybe could have a 400-year old granny in the form of a ghost loli. Which woud be kind of nice, but what we, people of culture, really want is the pureness of a loli, am I not right?
Coming to a conclusion, as science is still not ready to fully answer these questions, I propose a suggestion:
If you see a ghost loli, take her home.
