18-year-olds are also inexperienced in the travails of supporting yourself in life, and as a result of this, are much easier to sway with ideals rather than pragmatics.

It may be that it takes less effort to get you to sufficient fitness at that age, but it's nowhere near your peak. Unless you follow an elite athlete training program, it'll take you well over a decade of exercise to max out your potential. E.g. for muscle strength, most peoples bodies simply isn't capable of synthesising muscle fast enough in even a decade to max out your potential unless you use performance enhancing drugs.

You may have a point about average fitness and rate of improvement, though.

Based on my cursory research men peak physically in their mid to late 20's.

I'd say the peak strength must be 22-26, because 18-years-olds are really thin and lean (source: I love men ;)) – unless they've been practising musculation, but once again, if they practice musculation at 24, they'll be much bigger. But it takes well above 2 years to train a professional, so profesional recruitment takes them earlier. However, concerning draft followed by almost-immediate use on the battlefield, given this subjective data, it makes little sense to me, physically, to draft them so young.

It kinda depends on what kind of strength. For things where the athlete is rather lightweight/small and does some kind of fast/twitchy power moves you peak out somewhere before 30. But when all you need is pure strenght you have 30 to 40 year old world champions in many sports (you still have to start training when a lot younger to make it though) for example in power lifting where you have a lot of todays world champions born in the late 70s/early 80s.

One of the advantages (and disadvantages) of sending 18-20 year olds into battlefield is that if they die society spent less money/effort into teaching them some other valuable skill that would have been lost without getting the benefit out of it (due to the person being dead)