While the safety of a plane depends more on how it’s maintained by the airline than the number of years it's been in service, the age of an aircraft can play a pretty critical role in how comfortable you are while you’re on board. Older planes are going to be the ones that don’t have screens for in-flight entertainment, or the ones that have a poor resolution.
Older planes are also the ones that will leave you more dehydrated in flight (the cabins of older planes are made from aluminium, so humidity levels have to be set lower than in the newer models made of high-tech composites) which means you might arrive at your destination thirstier, with drier skin, and more jet-lagged.
To get the feature you’ll need a Silver membership to the service, which is about $2 a month or $15 a year. If you’re a frequent flier, then that year-long subscription makes a ton of sense. If you’re someone who only flies once or twice a year, then you might want to choose that month subscription instead or pass on the service entirely.
Anyone can use the site for free to see what planes are flying overhead around the world. To find out the age of your plane, you just need to search for the flight number using the site.
For instance, I often take Alaska 708 from San Francisco to Raleigh, North Carolina to see family.
A quick search shows that the flight typically happens in a Boeing 737, either a 717-990 or 737-890. Beside the model of the plane (that B738 or B739) is the specific model number. Clicking on that will show you more details about the plane.
For that particular route, it looks like Alaska swaps the planes out pretty frequently. For instance, the plane it used from the flight from RDU to SFO on June 29th was a 737-890 that is 11 years old
The same flight on July 1 was on a 737-900 that’s three years old.
The actual aircraft isn’t assigned very early, so this info won’t always be helpful for planning future travel. That said, you can get a feel for how an airline operates in terms of whether they always use the same plane or change things up for a particular route. Once you figure that out, you can typically see which planes are in the rotation.
And for upcoming flights, the info can be a great way to know ahead of time what kind of situation you’re likely to walk into.