At the half-year mark in 2012, I reported that the aviation industry appeared to be headed toward another significant drop in the annual average age for commercial jet aircraft retirements, with potentially big implications for aircraft residual values, financing options, parts pricing, lease rates and more. At that point, Aviation Week's Fleet database showed the average age dipping below 25.

Now the full-year results are in, and the average age did not end up below 25. It ended up below 24.

Whether the precipitous 2012 decline is an anomaly or the confirmation of a downward trend will matter a lot, and 2013 is shaping up as the year that will tell the tale.

First, the numbers.

A retirement-age analysis using Aviation Week's Fleet database shows an average retirement age of 23.2 years in 2012, down from 26.6 in 2011 and the lowest level in more than two decades. Retirement figures could yet change for 2012—and even for some of the earlier years—as aircraft now classified as stored are reclassified as retired after years of sitting, or some long-sitting aircraft make their way back into the operating market. But that is not likely to make a big difference in the trend.

The fleet data as of Jan. 17 shows 477 aircraft retired in 2012—126 widebodies, 317 narrowbodies and 34 regional jets—and the decline in the average age was not attributable to just one grouping. The average declined to 24.8 from 27.5 for widebodies, to 23.5 from 26.9 for narrowbodies and to 14.2 from 17.8 for regional jets.

Within the groupings, however, there were clear contributors.

For example, among narrowbody aircraft, the number of Boeing 737 Classics being retired jumped by about 40%, almost exclusively among the 737-300s and -500s. Moreover, they are being retired relatively young: under 24 years for the -300s and under 21 for the -500s.

The demand for -500s is less than for the other Classics because they are smaller—a particular disadvantage with the escalation in jet fuel prices—and none are being converted to freighters, which give aging passenger aircraft new leases on life. Marana Aerospace Solutions, a parting-out specialist, has another half-dozen -500s already lined up for this year.

Two other 737 models are being retired even younger: eight 737-600s were retired last year at an average age of about nine, and 11 737-700s at just over 12. The data also shows 10 MD-90s retired at an average age below 15—all of them Saudi Arabian Airlines aircraft that were purchased by AerSale.

Among widebody aircraft, Boeing 747-400 retirements doubled to 24, at an average age below 21. Airlines are replacing 747s primarily with the more efficient Boeing 777-300ERs. With the market for conversions of larger freighters having dried up, the prospect for a significant slowdown in 747 retirements seems slim.

The data also shows 767-300 retirements more than tripled, to 11 from three, at an average age of 23 years, and 767-200 retirements rose to 14 from six at an average age just over 24. The 767's future retirement pace will depend on what happens with Boeing's 787.

MD-11s also are starting to hit the part-out market: 10 were retired last year at an average age below 19 years.

But some of these individual aircraft trends are not likely to continue.

For example, the long-term impact of 737-600 retirements will be muted: Boeing has only delivered 69 of them (all between 1998 and 2005), and only 54 are still in service.

AerSale still has another 10 MD-90s it acquired from Saudi Arabian Airlines that are candidates for retirement in the next year or two. But Delta Air Lines is flying more than 50 MD-90s, most of the rest of the models in operation, and the airline has no near-term plans to retire them. In fact, it recently signed purchase agreements for 23 MD-80s for spare parts for its MD-88 and MD-90 aircraft.

Avolon, a Dublin-based aircraft lessor, argues that the recent decline in retirement ages will not last. It notes that Airbus A320 family and 737 Classic fleets are relatively young, so airlines retiring the oldest and least efficient of those aircraft are driving down the average. By the time the airlines finally retire the younger ones, Avolon says, most will exceed 25 years of age.

The Aviation Week Fleet & MRO Forecast also sees this trend ending soon. It projects annual retirement ages based on historical trends and other factors impacting retirement, including but not limited to projected maintenance requirements, aircraft operating economics, airframe versatility, and flexibility and ownership type. That forecast shows the average age skyrocketing for narrowbodies this year to 29 and remaining between 28 and 29.5 through 2018. It also forecasts a widebody average retirement age of about 27 in 2013 and 25-27 through 2020.

This year's results should show whether that forecast remains on track.