Barely matured whatnow? Optimus Prime's been the Matrix Bearer for nine million years, since before the Great War proper even started. He practically led the Autobots before there were Autobots. He was was our Megatron's archnemesis before he finally kicked the bucket. You sure you're not thinking of Rodimus Prime?

Wait, what do you mean "one of Ultra Magnus'"? Magnus is a wimp compared to Optimus, we killed him in five minutes. ...he got better, but it still counts!