Age alone is rarely a reason to walk away. It is a reason to know what is coming and when, and to price it.
Every mechanical system in a house is consumable. A twenty-year-old furnace is not a defect — it is a known future cost with a reasonably predictable timeline. The question is never "is this bad" but "when will this need replacing, roughly what does that cost, and is the price of the house reflecting it".
An older system in a home priced as though everything were new is a negotiation. The same system in a home already priced for its age is simply the deal.
These are general ranges rather than guarantees. A well-maintained furnace can outlast a neglected newer one, and manufacturer and installation quality vary considerably.
The most useful thing you can do with an inspection report is turn it into a schedule: what needs replacing in year one, year three, year five. That turns a list of worries into a budget, and a budget is negotiable.
The honest answer is that it depends on whether the market has already priced it. If comparable homes that sold recently had newer systems and this one does not, that gap is real and defensible. If every comparable home of that age has the same profile, asking for a full replacement credit will not land, because you are asking to be compensated for the age of a house you knew the age of.
Get quotes rather than estimates when it matters. A number from a contractor is a negotiating position; "roofs are expensive" is not.