There is no definitive 100% answer, but it is prohibited during Missions play, which some would take to mean it's not supposed to be allowed for game balance reasons.
Some GMs allow it, if only to suit a particular character concept. For instance, SR4 made it a lot easier to be a Rigger / Technomancer. But these days, it requires Submersion (TM version of Initiation) to purchase an Echo (TM version of Metamagic) in order to mimic a Control Rig's effects. So some GMs will allow it simply so you can play a "Dronomancer" from the start.
Some GMs do not allow it, viewing the overall concept akin to Archmages in fantasy settings. Undergoing Initiation, gaining access to the Metaplanes, and learning hidden mysteries of the arcane... aren't things that a fresh recruit who just hit the mean streets of Seattle should have under their belt. Remember, Standard level toons are people who have been Running long enough to "know the ropes" but not long enough to really make a name for themselves. So you can't really walk into the bar and introduce yourself as Merlin (and mean it). Now if you're playing Prime Runner characters, then it's more likely to have an Initiation or two under your belt by that level.