Stay away from aluminum tanks that are pre-1990. They may be fine but the LDS BS level is getting real high. Also, you may not want to deal with an old steel tank with the pipe thread valves. Nothing basicly wrong with them, just LDS BS about removing the valve.
If you are getting any used tank, insist on haveing the valve removed and having a look at it. If you see a lot of red rust (steel) or white corrosion (aluminum) make the sale contingent of cleaning/whip/tumble and if they are out of Hydro passig hydro.
The above is all contingent on price, I have picked up steel 72's and aluminum tanks for as little as $10-15 each. For that price, I will take a chance without looking inside as long as the outside looks clean. Cheapest I ever got was free at a tag sale at the end of the day and one I pulled from the metal scrap pile at the local waste transfer station (it still had 1,000 psi in it).
What are they worth? A newer clean 72 or aluminum 80 tank in hydro I'd go for about $50-75 and dicker from there. Check Ebay for the value of other tanks
As for age, well I just hydroed a 1960 made steel 72 that I got for $20. No problems and they gave it a "+" rate.