What is the oldest text/sutra of the Mahayana tradition? I'm often interested in the history of Buddhist texts and, being a Theravadan, I'm not very familiar with the Mahayana texts. Please help to educate me in this manner.
The oldest existing manuscript clearly identified as Mahāyāna is a birch bark manuscript of the Aṣṭasāhasrikā-prajñāpāramitā-sutra or Discourse on Perfect Wisdom consisting of 8000 [Lines]. This manuscript was carbon dated to  47~147 CE. A version of this text was translated into Chinese in 179 CE by Lokakṣema and was amongst the first few Buddhist texts translated into Chinese.

However it's clear that some texts were being composed at quite an early period, perhaps as early as the 2nd Century BCE. One contender for the earliest Mahāyāna text is the Ugraparipṛccha, translated by Jan Nattier as A Few Good Men (2003). It shows a very early stage of Mahāyāna in which monks who dedicated themselves to meditation were known as bodhisatvas and lived alongside Mainstream monks. 

Another serious contender is the Śālistamba Sūtra (translation published by N Ross Reat, 1993), which is clearly a little more developed than early Buddhist texts, but not yet a full-blown Mahāyāna text. It may well be considered a kind of "missing link" between the two phases of Buddhism. According to N. Ross Reat, the sutra could date as far back as 200 BCE.

Recently there were two very good articles on the very early Mahāyāna by David Drewes. See his academica.edu page "Early Indian Mahayana Buddhism" Parts I &amp; II.