>>102651502Cable's timeline isn't really a Point A -> Point B kinda deal. Consider this, the O5 being in the present only happened because of tampering with the timeline which shouldn't have necessarily happened. So a younger Cable from a different point in the timeline would have found out about the O5 being brought to the present while "our" Cable didn't know when he was that same age because it hadn't happened in the timelines he was jumping to.
Cable is untethered to the timeline which is how he can jump around it all willy nilly.
A page from Ewing's Ultimates says it better, but basically, the "past" and the "future" are fluid. The past changes (which we perceive as retcons or the sliding timescale) and any timeline has a bunch of "potential" futures that only get cemented as the timeline shifts forward from the "present" perspective. But then, because the past changes as well, the "present" present can change once it becomes the "past" present. There is no "set" timeline of events, so people like Cable or Kang who are essentially untethered from the flow of time can jump around. So this Young Cable came about because Cable at that age found out about the O5 while Cable at that age wouldn't have originally known about the O5 because they happened in a "potential future" which hadn't coalesced yet. The "present" we are in now was just one of many "potential" futures from Cable's perspective at the time, a future he might not have even been aware of.
It's also why bringing Cable back to life is trivial because Cable from earlier in his timeline can just find out that he died at a certain point in a certain timeline and just... not go there. Bam, Cable is back.