If it's time to replace your furnace, then it might also be a good time to have your ductwork inspected, too.
We tend to overlook these 'metal things' that hide behind our walls. They do a great job at keeping your rooms warm when you need it but they need some TLC, too.
The maximum life span of ductwork is between 20 - 25 years but at 15 years, they can start to deteriorate. It doesn't just 'lie there'. It expands and contracts over the year, causing seams to pull apart leading to leaks. This will lead to efficiency issues for your system.
Your ductwork was designed for your home when it was new but after 20 years or so, technology will most likely change. This can make your system require some improvements. For example, if you're installing a 'high efficiency' furnace, your installer will need to make sure that the old ductwork still meets with codes and will be the correct size for proper airflow. When you replace your furnace, consider your ductwork, as well.
Insulation is also very important. Condensation can form in uninsulated metal ductwork during the summer months and can cause rust. This can create holes which creates leaks and issues with connections. Regular maintenance will help keep these problems in check but replacement may still be needed, at some point.