I’m sure opinions differ, I’m offering my understanding of the matter, which is that a yana is based on one’s aspiration, and if one’s aspiration is to reach awakening not just for oneself’s sake but for others’ sake, then that counts as Mahayana.
This has the interesting result that Theravadins could be Mahayanists without using that term, and that “Mahayanists” could be “hinayanists” even if they call themselves Mahayanists.
Its not just my view by the way, the popular Vajrayana teacher Alan Wallace has said this:
“[Hinayana is not] Theravada it’s not some other school. It’s falling back into the aspiration, “I just want out myself. I just want out myself.” Whatever your monastery is, whatever color your robes are, whatever your lineage is and so forth, it’s falling back”
http://media.sbinstitute.com/courses/spring2016/05/#transcript
The usual practice, in my experience, amongst sectarian Mahāyānikāḥ in the know, is to direct polemics originally levelled against the Sarvāstivāda towards the School of the Elders, pretending them to be the same thing for the sake of a convenient pre-written shortlist of points of critique.
That would be quite misleading since the Sarvastivada Abhidharma and the Theravada Abhidhamma are quite different.