+1 that a bit of surface rust is no problem for your chain. I've rehabbed some chains that were seriously rusty, to the point that some links wouldn't move, and they worked adequately and lasted quite a while.

There are some good tutorials and videos on the internet that will help you figure out just how worn your drivetrain components are. Usually a cassette is good for 2-3 chains, and the chainrings are good for 2-3 cassettes (your mileage may vary). Cassettes are relatively inexpensive if you stick with entry level components. On many cranksets, it is also possible to just replace the most worn chainring (many riders tend to spend the majority of their time in one ring, leaving the other(s) in relatively good shape). Find out just how worn your components are. The age of the bike has little to do with it, miles ridden, style of riding and maintenance have far more to do with wear. A ruler or Park chain stretch gauge will tell you if the chain needs replacement. If the cassette is also seriously worn, I'd consider replacing it at the same time. Unless the bike has a whole lot of hard miles on it, chances are that the chainrings will be in decent shape.