An economist says most global population growth estimates are far too high, the data shows population peaking around 2060, and at 2.2 the global fertility rate may already be below replacement rate

If we get the lower end of these projections, you might plausibly live to see a world with a billion people less than now, but to see a world with less than a billion total you'd have to have an extremely long lifetime, which implies some medical breakthrough, in which case, unless you have some super-exclusive access, the population would stop declining, just get super geriatric. I suppose there are other ways to do get there, such as global nuclear war, but even that might let quite a few people continue in the southern hemisphere.