You literally can't determine if it's really old because that's an entirely subjective term. One thing is for certain, twenty years is abominably ancient as far as the salesmen are concerned. And it's true that an average system isn't going to last much longer than that. But "average system" is also a funny term that can be played with. Some systems last 30 no sweat. If you have a heat pump then sure, the outdoor unit would be pretty old at 20. But in my experience the indoor units of heat pump systems readily last longer than that. The biggest reason to change them is to match the outdoor unit (IF we're talking heat pumps).
In my opinion if your system is working well then don't think about replacement yet unless you just want to. There are benefits that include miniscule to major energy savings, less noise, better appearance that sway some people.
And as far as your past experience with that company goes, this trade has and continues to go through sweeping changes in how the business side is run. Few larger companies have escaped the corruption. They're turning all the service techs into salesmen because they know that the service tech has the most credibility amongst all of an HVAC shop's employees. They do so to the detriment of the service tech’s technical training. The sad part is that it's psychology 101 that the best personality for a service tech is almost never the best personality for a salesman. The two are incompatible. There are a LOT of well paid service techs who are technical hacks but keep their positions precisely because they bring home the new equipment sales bacon.
The best way to fight that is to stick with an honest technician and always request him. A company can change and its employee roster can be very inconsistent. But the character of a good technician is unaffected by that.
Speaking of bacon: With respect to gentleman above, watch out for salesmen that focus too much on indoor air quality. They can’t do as much about it as they’d have you believe unless you’re willing to spend considerable dough AND implement behavioral changes.