The official end of Greco-Olympian worship was in 380 AD/CE with the issuance of an Emperor's Edict. The very pro-Christian and rabidly anti-pagan Byzantine Emperor Theodosisus issued, "The Edict of Thessaloniki"-(in a Greek city approximately 100 miles Northeast of Mount Olympus). Unlike the earlier Constantinian based Edict of Milan which legalized Christian across the Italian peninsula and much of the Roman Empire, Theodosius' more draconian, Edict of Thessaloniki, essentially nationalized Christianity, making it the statewide religion of Italy, Greece, Asia Minor-(present-day Turkey), as well as
throughout many parts of the greater Roman Empire.
The nationalization of Christianity by Theodosius was also the official ending of centuries-(even millennial) old pagan institutions, such as the Theater, the Hippodrome-(Chariot racing), the Olympics (and possibly Plato's Academy, though I am not entirely sure when the Academy was officially closed). Theodosius' discontinuation of these age old pagan institutions, also included the ending of Olympian worship at the pagan temples, which were either destroyed or converted into Churches-(such as The Parthenon and nearby Temple of Hephaestus in Athens).
The publicly led evisceration of pagan institutions by Theodosius, was the official end of the Olympian religion-(though, I suspect that many Greek and Romans individuals,as well as communities, may have practiced their Olympian worship services cryptically, probably until the collapse of the Roman Empire around 476 AD/CE. By around 500 AD/CE, Olympian paganism would have been "out of fashion", even cryptically speaking, due to the ubiquity of Christianity within Medieval Greco-Roman societies).