For the heating pipes, assuming your boiler is in good condition and you don't have to add water constantly, black iron pipes should last a very long time. Ideally, in the heating system, you never need to add water. So, there's no free oxygen being added that's disolved in the fresh water to rust anything. It's when there's a leak, and you add water all the time to keep it working that you start to have problems with iron piping - it rusts from the inside. That's also why galvanized water pipes are a problem...the galvanized coating isn't perfect, and it's cut through at fittings, plus, the fresh water is constantly providing oxygen to let it rust. A vent is not generally wet, so often, they last a very long time, but not drain pipes that are wet and there's lots of moisture and oxygen to rust things out.