Start with the install year. Find it on the unit's data plate or in the original paperwork. That year is the starting point for the planning date, and it is the only figure every household shares. The rest of this walk-through is about what you add to it.
Write down how the system is actually run
Look at the thermostat schedule for the last full heating season and the last full cooling season. Note the set points, whether the house is held warm all day, and how many people are home during the day. A house with someone working from home, a bedroom kept at a steady temperature around the clock, or a thermostat held well above the usual setting asks more of its heating than a house that is empty for most of the day.
Do the same for cooling. A household with several people, a home office with equipment running, or a west-facing wall that takes the afternoon sun asks more of the central air conditioner than a quiet home with the shades drawn.
Check the shape of the house
Note the number of floors, whether there is a finished basement, and whether any rooms sit at the end of long duct runs. Rooms that never reach the set point do not mean the unit is failing. They do tell you the system is working harder for the same result, and that changes how you read its age.
Compare the adjusted picture with the guide
Open the guide for your system and read the typical service life and the planning notes. The heat pump and the gas furnace pages are written the same way. Then decide which way your household moves the date. A heavily used system deserves an earlier planning window and a closer look each season. A lightly used system may keep its service past the typical range, but you should still plan from the install year, not from how well it seems to run.
Ask these before you commit
Ask the installer or service provider what size and type they assumed for the household. Ask what load calculation they used. Ask whether the thermostat settings you described were part of that estimate. If the answers do not match the way you use the house, the replacement you are comparing may not fit the home you live in.