Nobody claims that 40 yeat old men are peal of physical perfection.
Car accident rates have an obvious confounding factor. Of course the people with +5 years experience driving have a lower accident rate than the people who just got their license. Skill comes from practice. If you pushed the legal age for a drivers license to 25, the 25-30 year old demographic would probably suck at it and get into accidents in their first ever years on the road. There's no way around the fact that driving judgement has to be learned at some point, and is not just bestowed by the maturity fairy when you hit the big three ooh.
Ability and willingness to follow orders is probably paramount, along with reflexes, and some common sense and logical abilities. And then of course, proficiency in your role.
18 year olds are typically right out of school, so learning new roles comes easy. They've also typically come right out of a situation in life where the expectation of following commands is pretty fresh, even if they perhaps weren't the best at following through (the military has ways to fix that).
And perhaps most importantly, for folks that are staying in for the career, or at least longer than the minimum level of service, joining at 18 means that you've had time to get training and experience so when you DO hit that "prime muscle strength" time of life, you're a well honed killing machine. Waiting til you've hit that point means you'll be going in cold, and get suboptimal results.
I'm going to take a stab in the dark and say by sheer coincidence you're in that 25-30 year old male demographic, and in about 5 years you'll be of the opinion that actually its 30-35 year olds who are clearly the strongest, toughest, most mature and capable fighting force and all them 25-30s are a bunch of idiots that don't value life and think with their dick.