As per Wikipedia, Mahabharata was initially composed around 400 BC. If Mahabharata was composed around 400 BC it should have some reference to Buddha / Mahavira who were born some where around 500 BC. Also an important historical event during that time was Persian invasion by Darius the great in 519 BC. There is also no mention of that historical event.Are there any sources which try to assert on the correct time line of composition of Mahabharata?
Like many of the great Indian epics, the Mahabharata was composed around 400 BC, as in written down. It was however orally passed down from a older time period.
Many people date the events listed in the Mahabharata, such as the battle at Kurukhshetra to Iron Age India, around the 10th century BC. I would agree with that, it might have elements going back into Vedic India, but the Mahabharata is around settled powerfull and stable kingdoms, which did not exist fully until the Iron Age. You can see this split in Vedic and Iron Age "Epic" India if you read the Mahabharata and the Rigveda.
The Mahabharata refers to kingdoms, and cities, and a very much settled population while the Rigveda, the oldest of the Vedas which dates back to almost 1500 BC, talks about nomadic warrior tribes and migration.