Definition of Purana and Itihasa
Puranas are by definition, the narrations of incidents that occurred in the remote past. The Itihasas (the epics) on the other hand, are by definition, the narrations of incidents that occurred during the life time of the narrator. Thus Mahabharata and Ramayana are records of contemporary history where as all the Puranas like the Bhagavata_Purana, Matsya-Purana, Vishnu-Purana etc are records of pre-history. But this doesn't in any way mean that the Puranas are older than the Itihasas. For example some one in 12th century can author an Itihasa about an incident happened in 12th century and somebody else can author a Purana about an incident happened in 15th century, in 21st century.
Can any incident mentioned in Purana be older than the incidents mentioned in an Itihasa? Yes. Incidents in Purana can be older or later or contemporary to incidents mentioned in an Itihasa. For example several Puranas mention about the flood during the period of Satyavrata Manu, that predates incidents of Mahabharata. Some Puranas mention about coronation of king Nanda in Magadha, which occurred several centuries after the incidents of Mahabharata. Nanda's coronation is also part of recorded history of modern times. The definition of Purana does not in any way indicate that incidents mentioned in a Purana are old; it only indicate that the narrator is referring to incidents which occurred several centuries before him.
Puranas embedded in an Itihasa
Besides this, Itihasas like Mahabharata and Ramayana contains many Puranas embedded in it. Hence, not all of the incidents mentioned in Itihasas are contemporary incidents. The life of the Pandavas since their military education as well as the Kurukshetra war are contemporary incidents mentioned by Vyasa in Mahabharata. But Mahabharata also contains narration of Nala & Damayanti as well as Satyavan and Savitri which are Puranas; these are added by others who lived several centuries after the life of Nala, Damayanti, Satyavan or Savitri. It is also probable that the narration about the submergence of Dwaraka, the final journey of the Pandavas, their death and Yudhisthira's mythical entry into heaven; these were narrated by a narrator who lived several centuries after the Pandavas. Some consider narrations about Pandavas birth and early days too as Puranas or as later additions. If we look at Ramayana, Valmiki the author was a contemporary of Rama. He was a sage who gave shelter to Rama's wife Sita and her sons Lava and Kusa after Rama discarded Sita and sent her out of Ayodhya. Narration of incidents about Rama's banishment into forest, his forest life, his battle with Ravana, his coronation as Ayodhya king, banishment of Sita into forest and Rama's union with his twin sons were all authored by Valmiki as contemporary history. However Rama's birth as well as death were narrated by some other Valmiki in the lineage of the original Valmiki as a Purana (or as pre-history).
Puranic (pre-historic) narrations usually have lesser credibility compared to Itihasic (historic) narration. Some times fables are added. Some times narrations about the birth and death of the central figures are exaggerated.
The narrator of a Purana lives several centuries after the incidents mentioned in the Purana where as a narrator of an Itihasa (epic) lives contemporary to the incidents mentioned in the narration. Because of this, the Vyasa who authored Mahabharata was contemporary to the incidents mentioned in Mahabharata, where as the Vyasa who authored Bhagavata_Purana lived several centuries after the incidents mentioned in Bhagavata. Life of (Vasudeva) Krishna is mentioned both in Mahabharata and Bhagavata. But the Vyasa who narrated about Krishna in Mahabharata was a contemporary of Krishna, where as the Vyasa who narrated about Krishna in Bhagavata Purana lived several centuries after Krishna. The Vyasa who authored Bhagavata probably was a Vyasa in the lineage of original Vyasa who authored Mahabharata.
Itihasa can grow
Itihasas can also grow in size when contemporary information is continuously added to it. This has happened to Mahabharata. That is why it is several times larger than Ramayana. In case of Ramayana this tendency of growth was not so prominent. The Santi Parva and Anusasana Parva has thus grown into huge sizes as several authors added contemporary information into these Parvas. Thus we could see complex definitions of caste system, conversations and religious debates between proto-Buddhists (who were often depicted as Nagas and some times as Yakshas) and Brahmanas in Santi Parva and Anusasana Parva. Another Parva that has grown thus is the Vana Parva. Based on the dramatic narration found in Virata Parva (which explains how the Pandavas lived incognito in the court of Matsya king Virata) some consider it as a later addition by imaginative folk artists who brought Mahabharata to the masses through their folk dance and folk drama. Such folk arts are still current in the form of Yakshagana (the song of the Yakshas) and Kathakali (the enacted (his)story).
There are always exceptions. Bhavishya Purana is a Purana, which has grown due to addition of contemporary historical information. It is believed that Bhavishya Purana was edited and material was continuously added to it till 17th century. There are also some excellent historical information found in many Puranas, but one must filter out the heavy maze of fables and imaginative narrations to extract these historical information. Similarly Itihasas contain fables and imaginative narrations too but they stand out due to the predominantly historical nature of these texts and these can be easily filtered out or analyzed for their historical cores. Usually the first and last portions of an ancient text (Vedas, Epics, Puranas) may contain later additions. Some times the first and last portions of a sub-section of a text (like Mandala, Parva, Kanda) too may contain later additions.