The animals with bilateral symmetry in young stage and radial pentamerous symmetry in the adult stage, belong to the phylum -

In phylum Echinodermata (starfish, sea urchin, sea cucumber, brittle star), the larvae (e.g. bipinnaria) are bilaterally symmetrical.
During metamorphosis they change into adults with radial symmetry based on five parts (pentamerous radial symmetry), e.g. the five arms of a starfish.
Molluscs and annelids are bilaterally symmetrical as adults. Cnidarians are radially symmetrical but not pentamerous, and their larva (planula) is not bilateral in this sense.
So these animals belong to the phylum Echinodermata.