Can't speak for Avenues; never even considered one. When I was shopping for mine, I was determined to get a Sprinter-based RV, but wasn't sure I wanted an Interstate until I compared it to the Winnebago ERA and Roadtrek Adventurous and saw the vast difference in quality.
As for Interstates, I believe the first ones came out in 2004. There are some AirForums members driving around 2005- and 2006-vintage Interstates, so I'd say that age of the vehicle shouldn't matter.
The Mercedes diesels are a tad bit underpowered for the size of the vehicle, but you'll not notice it much unless you plan to tow a trailer with it. More of a problem with older models that used the 5-cylinder diesel, less so with the 6-cylinder turbo-diesel in the newer models. To balance that out, the 5-cylinder does get better fuel economy.
Older models were based on the Sprinter 2500; newer models on the Sprinter 3500. The only functional differences are: 2500 has single rear wheels, lower GVWR, and smoother ride because of less unsprung weight. 3500 has dual rear wheels, higher GVWR, slightly rougher ride, but still not bad.
Some (former) owners have reported "wobbling" in the handling, which I suspect might be due to poor front-end alignment on their particular units. I personally haven't noticed any such thing on mine; it's stable as a rock. High profile does mean it catches crosswinds like a sail, but not enough to make you wrestle with the wheel, just enough to make you pay extra attention to staying in your lane.
As for age vs. value, all 2500-based Airstream Interstates are roughly comparable in terms of quality and amenities. Changes were minor and incremental— no major retooling. Ditto for the 3500-based Interstates. The biggest change between consecutive model years was the transition from 2500-based to 3500-based. So, once you pick either a 2500 or 3500, you can go with the oldest of that version still on the market and still buy with a reasonable level of confidence.
As with most RVs of any brand, the biggest drop in value comes in the first year, mostly as a reflection of the fact that dealer profit adds to the price but not to the value. So in the first year of an RV's existence, it depreciates by one year's wear-and-tear plus whatever profit the dealer made. All depreciation thereafter reflects ongoing wear-and-tear.
This means that even if you buy used for a one-year-old Interstate, you'll save a bundle over buying new. Consult the NADA Recreation Vehicle Guide http://www.nadaguides.com/rvs.
In the online NADA guide, only check off options, not standard equipment. Since most Interstates are fully loaded as standard, there aren't a lot of options to select.
Using my own Interstate as an example (everything standard except the solar panel), the NADA Recreation Vehicle Guide shows a 2012 Interstate with solar panel as low retail $95.3k; average retail $114.8k, MSRP $125.6k.
By comparison, a 2011 Sprinter 3500 with no conversion by Airstream (the base vehicle for my Interstate) shows an NADA base price of $40.4k. All the rest of the price of the Interstate is Airstream value-added for the conversion.
On the NADA listings, you can consider the "low retail" price to be a zero-profit price, or what the vehicle is actually worth, and anything above that represents seller profit. If you were to trade in an Interstate, you could reasonably expect to get 90% of the Low Retail price as a trade-in value if you're not being ripped off.
Note that the NADA listing for MSRP shows the original
MSRP, not the MSRP of the used vehicle today. For example, the NADA listing for a 2004 Interstate, no options, shows MSRP $76.7k, low retail $23.5k, average retail $28.3k. This represents the bare minimum price you can expect to pay for an Interstate, since thast's the oldest model year in existence.