I recently decided to keep my house in Denver for the long term (after 10 years of ownership). The furnace is the builder furnace and is 22 years old. I've never done anything to it in terms of service other than filter changes in 10 years. It continues to hum along. Most people around my neighborhood did not see this life out of this same model and dealt with expensive repairs and unplanned replacements....
Howdy Neighbor! If you were to sell, a home inspection would flag the age of the furnace. If it's a vertical airflow design with AC, the inspector would likely recommend replacement since the AC condensate tends to accelerate the corrosion of the furnace heat exchanger.
The two big failure are heat exchanger cracks and burner failures. Heat exhanger cracks are potentially deadly if you don't have CO detectors and you aren't having the furnace checked ever year or so at this age. A properly adjusted and maintained burner system could last 30+ years, but replacement parts become difficult and expensive to maintain at some point. Express shipping and multiple visits drive up the repair cost substantially. (Not to mention utility closets are rarely designed to leave adequate clearance for service / teardown!)
If you have an old school single stage furnace with a vertical flu, you will likely see a 20-30% reduction in energy costs with a new 90+% efficiency unit. This was an easy choice for me around 2009 when there were energy efficiency rebates available through XCEL and tax incentives.
Whatever you do, install a carbon monoxide detector or two if yours are over 10 years old or you don't have one. Consider doing the same on smoke detectors. The detector systems have a rated life, which is typically 10 years.