Which are considered to be the oldest suttas? Though I've seen dating of mahayana suttas here and there in passing with some analysis, I understand the pali canon has suttas of many ages. For example, one collection I've often heard to contain many old suttas is the Itivuttaka.

However, I've don't recall seeing any information about dating of specific suttas, or what criteria for estimating the age are used. Is there a set of suttas which has been identified as the possibly oldest texts?
I was told that the Sutta Nipata is a one of the oldest collections. I'm no Buddhist scholar so I can't help with any great textual analysis of the sutta. However Thanissaro Bhikkhu says here that the The Parayana Vagga is quoted in other (more recent) parts of the Pali Canon so this makes it a good candidate to be considered one of the oldest. To quote the relevant part of the passage


  There is evidence that these sixteen dialogues were highly regarded
  right from the very early centuries of the Buddhist tradition. [...]
  Most of the Cula Niddesa, a late addition to the Pali canon, is
  devoted to explaining them in detail. Five discourses — one in the
  Samyutta Nikaya, four in the Anguttara — discuss specific verses in
  the set