I can think of a few reasons but given that all other countries don't require it, I assume it's just one of those things that suck. Supposedly, voter registration lists are actually used to call people for jury duty, so in some ways the municipal governments are using those lists to determine what citizens 18 or older live in the area.

You fill out a form with your name and address, check a box saying you are a citizen and will be 18 by within a month of the next election, and then you mail it to the Board of Elections. So it kind of sucks that you have to do it, but it's not exactly an onerous task (you don't even need to pay for the stamp). Sometimes volunteers go places with forms, and you can just fill it out and hand it back to them - when I was a senior in high school people came to the school to offer us the chance to register, and university campuses and suchlike often have tables offering this.

You can also do it at the department of motor vehicles when you are applying for a driver's license or ID card, although the DMV screwed up mine up this summer (I've been registered to vote for over 20 years, but you have to re-register when you move, because polling places and local elections are different).

You do not have to register a party, ever. You may vote for anyone in any party without being registered to that, or any, party (i.e. you can register Democrat and vote Republican in a particular election). The the only reason for registering a party is that you are then allowed to vote in that party's primaries, the elections that determine who the party's candidate will be. I've never done it, but I presume that you can also change your party registration at any point - but party registration has nothing officially to do with whether you can vote (except in primaries) or who you vote for.

Different laws made at different times, and in the cases of sex and driving, these are state and not federal laws.

During the time of the war in Vietnam, the voting age was 21 (and the alcohol-buying age was 18). The draft age was 18, and there was an active draft ("You're old enough to kill/But not for voting..." - 'Eve of Destruction', circa 1965ish), so ultumately a lot of activism around that resulted in a constitional amendment changing the voting age to 18, which is how I've always known it.

I remember when the drinking age was 18, although I was much younger than that. Because of concern over drunk-driving incidents (and they do say those are down now, although there have also been a host of other initiatives to combat this), the age was raised first to 19 (in 1978, which I know exactly because Mr. Wildwoman turned 19 that year), and then 21 (in 1980, the year he turned 21).

'Age of consent' (it's not illegal to have sex if you are under the age of consent, it is illegal for someone over the age of consent to have sex with you) and driving ages are laws made by the states, so they are not identical all over the country, but tend to be similar. Age of consent is usually 16 or 17, but if you're young enough for this to be relevant, checking your state is wise. Getting a learner's permit was 16 in my state (New York) when I was 16, and probably still is. Learner's permits and driver's licenses are issued by the state, not the federal government. Rural states tended to have younger ages (this may have been evened out now, but I don't know), because licenses are required to drive farm equipment like tractors. Driving isn't really considered a privilage of adulthood, but a privilage of almost-adulthood.

Thanks WW, this cleared up a lot for me!