There's a few things here to unpack, but I'll clarify what I meant by "You draw for decades."
As you mentioned, there are 16 year old prodigies out there with insane skills. However, while they may not have been drawing for as long as someone in their 30's, those youngsters likely have been drawing lots more than the average child all through their childhood. In terms of the number of hours practice racked up, they'll still have heaps. Plus, you learn differently as a child than you do as an adult. Ever heard of the '10,000 hours to mastery' thing? A 16 year old prodigy started that early, or they're one of those people who just need fewer hours to reach that level. Just as some people need more. Which leads me to my next point...
Talent is not a myth. I'm a music teacher. I see talent in action all the time. Talent is not the only thing which will get you to a professional level, and hard work is waaaaaay more important, but some people do simply have a higher base aptitude for some skills than others. (And therefore have an easier time mastering those skills.)
It's why I teach some 12-year-olds who can sing everything you throw at them, and some 12-year-olds who struggle with basic pitch and rhythm. They've both been singing since childhood, and they're both surrounded by music. If I assumed the latter wasn't working hard enough, rather than accepting that they're coming from a different starting place, I'd be failing at my job. Instead, I accept that music may not come as naturally to the latter, and we approach their learning to sing in a different way, which structures their improvement. Sometimes, the latter eclipses the former, through lots of hard work, time, and dedication. Especially if the talented kid just coasts along on natural ability, and doesn't put as much work in. (Very common!)
Talent means less in art than it does in music, because art is a more mechanical skill. (Pitch and rhythm can be taught, but they're far more innate.) But it still factors into how fast one learns, and is especially pertinent when speaking about 'prodigies'. Talent does not negate the amount of hard work which goes into learning how to draw, I cannot stress that enough. And it is definitely overemphasised in importance by people who haven't cultivated artistic skills, and don't understand how they work. But it does exist.
As for as a back door into the skill you're asking about... it really is just practice, but there are a few things you could focus on practising in order to turbocharge that improvement. Gesture drawing is one, ensuring that each line you use to draw a figure is smooth and well-placed, rather than lightly sketched out as a bunch of small lines. Practising figure drawing in that way, over and over and over, will provide you with the line confidence and visual library you need to pull this off.
Also, starting to deliberately omit small details from your sketch, so you can practice drawing them in lineart. Start with little things like belt buckles, pockets, hats and so-on, before moving up to more complicated things. (I'm constantly forgetting to sketch one character's hat, and usually just chuck it in at the lineart stage.) Between those two bits of homework, you'll find yourself better able to quickly visualise where a line should go, and accurately lay it down.
Hopefully, that clarifies what I meant a little bit.