“Peak X” is a phrase that is often used to convey the highest point in a long term trend. And if we are past that peak, that implies that the trend will continue to be lower and lower for the foreseeable future.

Consider the nation’s supply of 18 year olds that would normally enter college. We can’t just create more 18 year olds on demand, couples would had to have gotten busy 19 years ago to produce today’s supply of 18 year olds. With birth rates declining, not only are there fewer 18 year olds now than before, but those 18 year olds will be able to make fewer humans when they all get busy with each other later.