Judge the age first. A system's age sets its place in the service life range, and you cannot compare two units fairly until you know when each one was made and when each one went in.
Read the data plate
Find the data plate on the cabinet, the side panel, or the inside of the access door. It lists the model, the serial, and often a manufacture code. Copy every line exactly as printed, including letters that could be mistaken for each other.
Decode the serial with the maker's rule
Many makers use a serial that encodes a manufacture date. Each maker sets its own rule, and the rule can change over time. Ask the maker's published decoder or support line to explain the code for your model family. Do not decode from memory or from a forum post, because a wrong read can move the age by years.
Keep manufacture and install separate
A unit can sit in stock before it is installed. The manufacture date tells you the earliest the unit could have been in service. The install year is the one that drives your planning window, so record both. Use the install year from the receipt, permit, or service tag wherever you can find one.
Check the records and ask the right people
Ask the seller or landlord when the system was installed, who installed it, and what repairs it has had. Ask for any warranty or service papers still on hand. Match the model family on the plate against the system guide on this site, not against a feature list or a sticker claim. Once the date is pinned down, compare systems on age and condition, not on how they were described. The appliances ranked by typical service life page is a useful reference once you have the date in hand.