You can determine age by reading the label. Go to Know-how, reachable from the site map and read the whole page. It will tell you how to read the label and also how to do an external inspection of your heater.
Old water marks and rust on top of the tank may not be significant. Actively wet, rusty places would be more ominous. Likewise, if you remove the element ports (after TURNING OFF the power) and find water in there. If not, though, a new anode might keep your tank going. Or not. There are many situations. We don’t want people to try to maintain something on its last legs, but neither do we want them to throw a tank away just because it’s a certain number of years old. In some places, water heaters can last a very long time with little maintenance.
Anyway, as to anodes, they are either magnesium, as with the Rheem and Sears, or aluminum, which the Lowe’s will have. You can read our feelings about the two metals elsewhere on the site. There are no stainless steel or copper anodes.
As to water heaters, we make no specific recommendations. Rather, we have two sections, The Right Water Heater and The Best Water Heater. Read those, then take the information and shop around based on that plus price plus energy efficiency. R values are gone, replaced by Energy Factor, but thick insulation is still a good thing and you may be able to measure at the drain valve or temperature/pressure relief valve, if it is on the side. One inch equals R-8, two inches R-16.
Hope that helps.