The Mahayana resulted from a schism within the early sangha after Buddhas death. They have the right to call themselves what they wish. Since the disagreements were primarily due to differences regarding the Vinaya Rules for Monks, I see no problem so long as they do not change Bubbha's core teachings re. The Four Noble Truths and their supporting factors: impermanence, dependent arising, kamma, and emptiness.
What Makes an Elder? :
A head of gray hairs doesn't mean one's an elder. Advanced in years, one's called an old fool.
But one in whom there is truth, restraint, rectitude, gentleness,self-control, he's called an elder, his impurities disgorged, enlightened.
-Dhammpada, 19, translated by Thanissaro Bhikkhu.