Did you know? Most planes are only used for 25 to 30 years before they are put into retirement. But the truth is, planes are designed to last much, much longer. Nonetheless, they are booted from service because the cost of maintenance increases, as well as the criteria for inspection, become stricter as the plane ages.

According to a US pilot and writer, Patrick Smith, airplanes are traded, disposed of, or sold “because they’ve become uneconomical to operate”, and not because they are breaking down.

When planes are designed, they are created for specific markets and jobs. As soon as the aircraft begins to underperform, the airline loses money. As a result, it’s passed on to another carrier (i.e. low-cost ones) that can make a profit from the older airplane since their routes and requirements fall in line with the older aircraft.

As technology advances, especially with new engines becoming even more efficient than its predecessors, airlines are on a constant move to switch over to newer planes that are cost-effective. Moreover, newer jets produce much less noise, which is also more appealing to passengers.

So where do some of the biggest air carriers rank in terms of age of their fleet? Interestingly enough, Russia’s air carrier Aeroflot has the youngest fleet, averaging about 4.2 years, followed by Hainan Airlines at 4.9 years, and then China Eastern Airlines at 5.3 years.

On the other side of the spectrum is Delta, which has the oldest fleet of planes in the world, averaging about 17 years, followed by Air Canada at 14.2 years, and then United Airlines at 14.1 years.

Some of our favorite airlines such as Emirates, averages at about 5.4 years, Etihad and Qatar at 6.1 years, and Qantas at 9.7 years.

Looking at the list of major airlines in the world (below) and their average age of jets, North American and European carriers usually have the oldest planes, whereas Middle Eastern and Asian airlines have the youngest.

1) Aeroflot: 4.2 years
2) Hainan Airlines: 4.9 years
3) China Eastern Airlines: 5.3 years
4) Emirates: 5.4 years
5) EVA Airways: 5.6
6) Saudi Arabian Airlines: 5.9 years
7) Qatar Airways: 6.1 years
8) Etihad: 6.1 years
9) Air China: 6.3 years
10) Ryanair: 6.5 years
11) China Southern Airlines: 6.9 years
12) EasyJet: 7.2 years
13) Cathay Pacific: 7.7 years
14) Singapore Airlines: 8.1 years
15) LATAM: 8.1 years
16) Japan Airlines: 8.7 years
17) Korean Air: 9.2 years
18) Jet Blue Airways: 9.3 years
19) All Nippon Airways: 9.4 years
20) Thai Airways: 9.6 years
21) Qantas: 9.7 years
22) American Airlines: 10.3 years
23) KLM: 10.7 years
24) Southwest Airlines: 11.5 years
25) Lufthansa: 11.5 years
26) Air France: 12.6 years
27) British Airways: 13.2 years
28) United Airlines: 14.1 years
29) Air Canada: 14.2 years
30) Delta Airlines: 17 years

(This article first appeared on Traveller.com.au.)