I'm thinking that it was a typo, and that you're correct in your hunch that they meant to type in 5'4". 6'4" would be awfully tall for a 14 year-old boy, especially since boys tend to have their growth spurt later. Certainly my memory of jr. high and high school is that a lot of girls towered over most of the boys until about late 10th- or 11th grade, when the boys suddenly shot up in height almost overnight. Also, 6'4" and 150 lbs. would be pretty thin, and in the Charley Project/NamUs picture, there's a kind of soft fullness under his chin. Nothing so much as to be overweight, but I don't think he'd have the roundish cheeks and under-chin fullness at 6'4" and 150 lbs. I know that it says the weight is just an approximation, but even with 10 or 15 lbs. either way, give-or-take, the ratio of weight-to-height would be worth remarking on as a descriptor. Mr. Mouse is 6'3" and 165-170 lbs., and if I'm supposed to meet him at a restaurant or someplace, all I have to say is, "Is my husband here yet? He's really tall and really thin," and everyone will immediately know exactly who I'm talking about.
One other point: as someone who was alive in the 70s, I will say that there's not anything particularly unique about the boy's hairstyle, in either picture. That deep part/long side-swept bangs look was pretty common in the 70s--Robin Gibb of the Bee Gees had that same style for much of his 70s career, and if I dug out my old class pictures and jr. high yearbooks, half the boys would have that same hairstyle. Remember that blow dryers and curling irons didn't become popular until the mid-70s, and things like hair gel and mousse weren't really widespread in average-town, USA, until the early 80s. If you had some body and wave in your hair, there really wasn't much you could do (especially for a guy) other than either buzz-cut it, let it frizz, or grow the bangs out enough to weigh some of the curl down. Doesn't mean that it's not the same boy in both pictures, just don't put too much weight on the hairstyle as identifier.
ETA: I've been watching the Costa Rica vs. Netherlands game (World Cup) for the past couple of hours, and the commentators can't stop talking about how tall the Dutch goalie is--in fact, he's a substitute goalie who was brought in for the shootout at the end specifically because at 6'4", he's several inches taller than the original goalie, and all the other players on the field. So yeah, if they're carrying on about a grown man being 6'4" (and statistically, Dutch men are the tallest in Europe on average to start with), then I really don't see anyway that Darren was 6'4" at 14, at least not without it being a medical condition of some sort. Might help in narrowing down any unidentified remains, or maybe the "captive boy" photo can be measured for angles, size of head (or whatever), and some forensic specialist could estimate height from there.