This could be due to differences among tissues in synthesizing and accumulating different compounds, which is also different with age of the tissue. Some compounds produces by some tissues could be repellent or toxic at young stage than when the tissue becomes older.

This is essentially an ecological question.  Parasites are adapted to seek, identify, and survive in an ecological niche.  

Parasites often use chemoreception, detecting anything from pheromones to metabolic by-products (e.g., mosquitos follow a carbon dioxide gradient and catfish that parasitize the gills of other fish follow an ammonia gradient) to find an appropriate host and host tissue.  Protozoal parasites may find their way to the appropriate tissue/cell type by detecting host chemokines or other molecules.

Importantly, some parasites actually infect a wide array of different tissues (non-tissue-specific infection) but are usually only able to persist or reproduce in a subset of those tissues, because the host immune system has the advantage and clears the parasite infection from the other tissues.  This appears to be the case for the microsporidium Pseudoloma neurophilia, a parasite of zebrafish.  Larval and juvenile zebrafish are very susceptible to infection in many tissues, but the parasites persist in immunopriveleged sites, such as the hindbrain, spinal cord, and oocytes.