The world's oldest religion, at least chronologically from a historic perspective is Hinduism (Sanatana Dharma). However, please keep in mind that everyone of every religion considers their religion to not be 'new'. Christianity is seen as a fulfillment of prophecies based in Judaism. Baha'i is based upon prophecies in Islam. So as far as Judaism, Christianity, Islam, and Baha'i, if you ask a member of this religion they can each point backwards through the Abrahamic traditions to how each 'fulfilled' other areas, thus keeping them within the same line. The same can be said of Buddhism, Jainism, Sikhism, etc. So when asking this question it is best to be aware of what exactly you are wanting to know. As one Christian I know said "Jesus was a part of God in the beginning, thus Christianity is the oldest religion because Jesus was always a part of God, but it is young if you only compare it from a historic chronological perspective." I think that is well said.
Thus from a Historic Chronological Perspective from oldest to youngest the religions go:
- Hinduism
- Zoroastrianism
- Judaism
- Jainism
- Buddhism
- Taoism
- Confucism
- Shinto
- Christianity
- Islam
- Sikhism
- Baha'i
- NeoPaganism*
Please note that many in the NeoPagan movement claim direct connection to the Pagan past and often direct it to a specific ancient culture/civilization, but not all. Some state that theirs is a new religion based upon a revitalization of ancient religions, but not the ancient religions themselves. So one has to be careful in this area, too. Native American beliefs and Shamanism are not listed, nor did I name Unitarian-Universalism, Rastafarianism, etc. I mainly listed major world religions. (UUs do not necessarily view themselves as a religion in itself, but rather an approach to religion with many in a UU congregation holding a wide variety of beliefs from many of the worlds religions. There are some UU, though, who would claim that as their religion so no offense to those who do).