A couple of decades ago, I went to an exhibition at Jain Samaj Europe entitled 'Jainism - The Oldest World Religion'. Whereas it is probably the case that Mahavira was around before the founders of other faiths, this is a perspective perhaps relevant to the doctrine of anekantavada as found in Jainism in that it will tend to look outside itself and assert that nobody has the full objective view on anything, so it might look at itself as having been founded many thousands of years in the past by Mahavira just as Zoroastrianism, for example, was founded a bit later. Even so, to me this reflects a misconception.
Take Biblically literalist evangelical protestant Christianity for example. In its case, there seems to be a world view that the Universe was created around six thousand years ago and God put the first humans here soon after, at which point all hell broke loose and it became necessary to heal the rift between humanity and divinity by working towards God manifesting as a fully human, fully divine unique being who could take on human sin. From the perspective of that view, which is not in my opinion the only viable Christian belief system but it serves well as an illustration, Christianity is automatically the oldest world religion according to it in this form because it offers a world view which dictates that what it believes had to happen to save the human race, and it happened at the beginning of the world.
Similar things could be said, for example, of Shinto and Hinduism, except that in those cases it's more that they were "always" around. That is, they have no founders, and are more traditional cultural practices which have latterly been labelled as religions. In fact, in a sense the two of them are almost the same religion, as is the observance of Olympian deities, Roman divinities, Odin and Thor and the like.
At this point, the words "looking at it from the outside" could occur, but they won't because there is no outside. There are such things as history and anthropology, and they might ascertain that, say, particular practices in a particular faith seem to have begun at a particular time, but the problem is that those very disciplines don't exist in a vacuum. Somewhat scientific approaches to things arise out of a particular tradition, maybe that of the "Peoples of the Book", i.e. all those cultures influenced by faiths related to that of Abraham. Hence it isn't really outside anything when something like that is claimed.
Even with Buddhism or Scientology (including the Free Zone), both of which seem to have definite founders, it's more discovery than foundation. Someone discovered that the Noble Eightfold Path was key to eliminating suffering or that E-meters were a good way of eliminating obstructions to psychic abilities. They also have their own cosmologies and histories.
Therefore, a claim about the origin of a particular religion doesn't really apply to the religion itself so much as the date at which it began to be labelled as such. Saying that Christianity is 2018 years old because that's when Jesus was said to have been born is a bit like saying that dinosaurs lived in the nineteenth Christian century because that's when their fossils were first identified as such. Faiths do not have an age. They reflect a particular view of the world from which there is no outside from which to judge them. The same applies to science.