Hinduism is a pagan religion to many christians in that it is a non-christian religion that has many gods, like the ancient Egyptians, classical Greece and Rome, and the Nordic religion of the Vikings.
The origins of the Hindu religion are probably not as old as Mesopotamian and Egyptian religion. Minoan and Greek religion also probably predate Hinduism. Roman and Nordic religion probably had there origins somewhat after Hinduism.
Primitive religion predates all these religions having its origin with mankind's first religious instincts in the early stone age of human prehistory. Shamanism among Siberians, Inuit, and Native Americans (among others) is a survival of ancient primitive religion. Primitive religion reflects humanity's innate religious impulse and is probably the truest and most natural of human religious types.
There is some interesting conjecture that late stone age pagan religions and especially the mideastern Judeo-Chrisitan & Islamic religions are strongly influenced by an alien presence on earth beginning just before the advent of agriculture and extending through the period of Sumerian supremacy. That sounds bizarre, but some have made a strong case from evidence in the Sumerian mythos.
I would say that Hinduism is the longest surviving, and still vital, pagan religion. If you need some dates for a paper or something, let me know and I'll get you the best dates I can find for the Rig Veda and other early religious scriptures and mythologies, including the length of the oral traditions that may have preceded the earliest written works.
Samm Dickens
(Bran th' Blessed)