They don't need to know what any one individual will do, they have data about e.g. how and when people start smoking. If you have, say, 70% of long term smokers beginning to smoke when they're 18 (or, out of highschool), then upping the age limit will hopefully diminish the number of long term smokers (i have no idea what the real numbers are of what the true motives behind the proposed changed were, this is just an example). Which, again, to make it clear that this isn't about whether any one individual, or a universe of individuals, can make good decisions (and neither is "good decision" a metric anyone should be using when trying to affect social change), their interest is people smoking less.
Voting is a completely different thing, there is (or ought to be that is) an interest in ensuring that every individual has the ability to participate in the democratic process, regardless of what they might choose. Voting is an unalienable right, and the less obstacles there are the better because any policy that affects one's right to vote is infringing on an unalienable right, as any suffragist would tell you. They don't "trust you" with the right to vote, they have a mandate to ensure voting rights and any change is and should be under immense scrutiny. Not that they are exactly good as they are, but others can expand on that if they want.
The military, well, i'm a very anti militaristic person so i don't want to touch that subject here, and there's many talk of malpractices concerning the military in the US. But, were i to expand on it, i would again demonstrate that it is a unique problem, and related regulation would again have no bearing on whether a person is intelligent enough, or makes good decisions (before being tested by the military on whether they have the necessary skillset that is).