My in-laws (both now 70, last trip 3 years ago) have done the trip to Australia a number of times but Lauren's right - the plane trip did take a hell of a lot out of them, so the last time they went we insisted they flew business class and it was fantastic for them - they both were able to sleep a good few hours, really nice food (m-i-l is diabetic and they catered specifically for her) - it made for a really good holiday for them. They got off the plane on both ends with practically no jet lag. Also they got to use the Business lounges in Dublin, Heathrow, Singapore and Melbourne which made for the breaks being quite relaxing too.
If Business class is too expensive - try looking at BA's World Traveller Plus option - my wife and I flew to Melbourne on this class a couple of years ago and the extra leg room, pre-allocated seats, extra recline angle, extra baggage allowance made for a good flight too.