It is normal to see older planes (20+ years) quite frequently. In fact, a few months ago I saw a plane over 50 years old (not in a museum but on a runway, about to take off).
Comparing to cars, there are not many cars on the road which are older than 10-15 years.
What are the reasons for this besides the following?
- Airplanes are very expensive. A new 172 would be over USD 300,000, and a thirty year old can be around \$50k or less.
- Airplanes follow a very strict maintenance schedule and are inspected thoroughly (ideally speaking). Hence their life is increased.
- Pilots are very well trained compared to car drivers, so they know a lot more of what they are doing. Hence they are more aware of the condition of the plane.
- Cars crash a lot more often than airplanes.
It appears that this holds true for both general and commercial aviation.