Because people often question this, I thought I'd give my thoughts.
"Why is old Cap there?"
I think in the timeline we have been watching, Steve Rogers was always there twice. Just from the way the Ancient One described things, I think we've been viewing old Cap's timeline this entire time. He never revealed himself, and even got Peggy in on the gig. Who knows? Maybe he told Sharon about it, too. (By the way, R.I.P. Sharon X Steve)
"Why didn't old Cap help out with, you know, Hydra and stuff?"
This one is actually a lot easier to answer, though it's still up to speculation.
A: He did, we just never saw it. If he just showed up after he disappeared in First Avenger, he could easily have helped out under a different guise (Nomad?). Something much more hidden, so no one knew about it. Or, perhaps he just re-enlisted with a new name. He would be, what, ninety during the events of Avengers and so on?
B: (the one I like more) Everything had to be perfect for "the One." If Cap even knew about time travel and the Quantum Realm before the right time, everything could have been lost. Imagine if Cap knew about time travel, and attempted to do it before the five years when Ant-Man escaped? He could have died, which would have resulted in something really, really bad, seeing how important he was in that movie. Therefore, Cap had to remain in the shadows while his younger self saved millions of lives.