Is it possible that you're either just perceiving this "cartoonish" look on your younger faces due to the fact that some stylistic choices which we associate with cartoons (larger eyes, simpler silhouettes, etc.) are actually a natural distinguishing aspect of youth as opposed to age? Or, further, that you actually make such stylistic deviations unconsciously due to the fact that those subjects actually do possess those variations, and you're just unconsciously exaggerating those bits just a little bit more than is accounted for by the age difference? I don't want to discount what you've specified, that this occurs even when you measure perfectly and nail the shapes and proportions, just thought I'd throw out those considerations as possibilities. I would try to use a process of elimination to work through all of the possible reasons this is occurring; suppose you eliminate shape and proportion issues, what else could it be? Perhaps you could use finer/subtler mark-making, as it makes sense that bold and sketchy technique would lend itself to the weathered visage of an aged person, while a younger face would be better served with more careful, gentle linework. If you do some studies and find that making that change doesn't positively affect your results like you're looking for, move on to the next item for elimination, and so forth. Iterating on the same subject while only making limited changes to your process each time might help you isolate what it is that's causing your dissatisfaction. I hope you're able to puzzle it out!