Generally, when buying oil, it's not the age so much the amount of km's covered. Lots of new cars nowadays recommend 10W-30, which has an ILSAC rating - meaning it’s a low friction oil - or an oil suitable for higher fuel efficiency. Manufacturers will recommend this oil (for general day to day running) up to around 100K. The tradeoff is the fact that the lower viscosity will mean lower shear thickness of oil (aka, there will be a thinner layer of oil protecting the parts in your engine). Another general rule is that the last number, whether it be a 30, 40, 50 or 60 (sometime even a 70, although technically this cannot exist as there is no SAE rating for a 70 weight engine oil) will be the viscosity of the oil at operating temperatures. Thus, no matter what the first number, at running temperature, a 10W-50 will be the same as a 15W-50 or a 20W-50.
Also, you will find oils with a greater variance in viscosity are more likely to be synthetic oils. Synthetic oils are considered better than mineral base, due to the fact that they are made with fewer impurities, which can afflict mineral base oils. These impurities can lead to an increase in oxidation of the oil (where it turns to black and stops being as effective in lubricating.) A synthetic oil therefore means that because the oil is purer, it is less susceptible to oxidation, thus being cleaner come time to change oil. Theoretically, this means that the oil will last longer, but it’s not a recommendation by oil companies to extend oil servicing, as most people neglect to change oil frequently anyway. Most European cars (with service intervals of up to 20K) will strictly recommend synthetic oil for use in there vehicles.
Generally, I would recommend running a lower viscosity oil (on start-up) then say go to a larger number for running for a car with 55K. Something along the lines of a 15W-50. this is due to the fact that the thinner the oil is on start up, the quicker the oil can get to the top of the engine, which will allow the oil to heat up quicker (as a low viscosity oil will also be able to absorb energy much quicker than a higher viscosity oil) which will in turn get to running temperature quicker. Obviously, as mileage increases, you should increase the thickness of the oil. As we speak, I use an HPR15 (15W-60) in my 4AFC, with 340K on the clock, and haven't had any serious consumption issues to date.
--for the record, I am an impartial employee of Supercheap Auto, and did a little study in this area