First off, the arrival of an ice age doesn't happen on schedule--it's like the first snow, it varies. I would say probably sooner than later. People studying the antarctic ice have noticed that an ice age occurs almost immediately after a period of global warming. The higher the amounts of carbon dioxide in the air, the faster the warming, the colder and longer the ice age tends to be.
Yes, 13,000 years ago (and another 100,000+ years before that) we were cave people, hunting mammoths and whatever else. So ice ages are nothing new to our species, we are very smart that is how we have been able to make it this far.
Agriculture is only about 10,000 years old, we have been able to grow crops only during this warm period. Without the crops, we will have a hard time feeding the amount of people we have currently on this planet. We may have to be cave people again.
The good news is with our technology we will see this coming and if we are wise, we will be able to prepare for this. If the ice age is as bad as "snowball earth" --millions of years ago, when the ENTIRE PLANET was deep in ice, we will have to live in fully contained environments like we would on Mars or the Moon (except we could still breathe the air of course) as going outside for even a few minutes could result in death.
I doubt we'll see it our lifetime, but we may if we trip some feedback loop on our planet with our mindless dumping of greenhouse gases in our skies. When it comes it could be a drastic change, we could find ourselves in a full-blown ice age within a decade or so. Every seen the movie "The Day After Tomorrow"? Great film, a little more extreme than what we would expect but it is a disaster film, nevertheless, I highly recommend it.