"No Taxation without Representation" is a quote I remember from school relating to the American War of Independence, and I think still has some relevance. I think that the age of responsibility should be set universely at 18, and in return, no-one under 18 should have to pay Income Tax. Likewise, on all buses and trains, children's fares apply up to 18, and children's rates for other things, like dental charges, should continue to 18. This does mean that alcohol etc. cannot be bought under 18. Also that voting rights are only conferred when you can pay Income Tax (yes, I know that tax applies in other areas such as VAT, but Income Tax is more 'personal'). Further, you will not be able to drive until you are 18, but since you would still enjoy cheap bus and train fares, is that not a good compromise, and car insurance at 17 is hardly cheap.
Having a standard age surely simplifies things, and 18 is a good compromise.