> consider how many years of experience a "senior" is typically expected to have

That entirely depends on what the experience is towards. If it is something like farming where you only get to experience a different scenario once per year due to worldly constraints, then one would expect many years — decades, even — before considering someone "senior".

But when the domain allows experiencing a new scenario every handful of milliseconds, you can shorten that tremendously. In that case, a couple of years is more than enough time to become a "senior" even with only a modicum of attention given to it. If you haven't "seen it all" after a couple of years in that kind of environment, you're never going to become "senior" as you are hardly engaging with it at all.