It seems like cars in F1 and before F1 are getting easier to drive and safer, hence the value of experience is getting to be less and less.
Well, they're certainly getting safer, and more capable, but easier to drive they are not (although having single make series, as opposed to what it was like back in the day, does mean there aren't "badly" handling chassis). There are two other factors that maybe explain the trend towards younger drivers.
Firstly that teams don't want to miss out on the "next big thing", they don't want another team to sign someone up who turns out to be the greatest ever, and therefore miss out on titles, just because they wanted to wait and see whether the driver would be good enough, rather than taking a bit of a gamble and putting them into an "affiliated" lesser team.
Secondly, and probably more importantly, these days the vast majority of drivers have been racing since thy were 5, by the time they're getting into F1, they've already had 15 years' experience. There has been an ever increasing trend towards more and more children, and younger and younger children taking up racing in a big way, seriously. So not only does the average age that they begin keep decreasing, but also, because of the increasing amount of competitors, the quality of the competition further down the age rage increases, and therefore so does the quality of the racing education throughout the age rage and various formula. And this trend just keeps escalating, on, and on. As such it's ever more likely that drivers will reach their maximum talent potential at an ever earlier age. They only then need to be able to be physically strong and fit enough to be able to cope with F1. It's better to get them into F1 where there's an opportunity as soon as they attain that level, so that they can learn the nuances of F1, and hone their skills further specifically for F1, rather than to wait.
Well, that's the logic as I see it anyway.