Rear deraileur maintenance / adjustment I have a 9 year old road bike with Shimano 105 groupset (approx 20 000 miles).  I clean the deraileur as best I can but have never taken it apart or replaced anything in it.
Recently, when I use the small chainring, the top jockey wheel seems to rest on the chain and cassette.  It makes some noise but shifting is pretty much unaffected.
Are you able to give me an idea of what the issue might be before I start disassembling things or ordering new parts?  What the expected lifespan of a deraileur?
As mentioned in the other answer, the first thing that could be tried is adjusting the B-screw.  This adjustment will vary the gap between the jockey wheel and cassette. Clockwise rotation of the B-screw moves the jockey wheel away from the cassette, widening the gap between them.  Conversely, loosening the B-screw counterclockwise, will move the jockey wheel close to the cassette.  Proper position for setting the B-screw is when chain is on small chainwheel and large rear cog. Measuring from tooth tip to tooth tip there should be a 5-6 mm gap between jockey wheel and large cassette cog. An Allen key of 5mm should just squeeze through the gap is how I determine my adjustment. Even though your shifting seems unaffected, the jockey wheel should not interfere or touch the cassette in any way.