As the mother of a kid in this age group who is interested in bonsai, I'll toss out the things I've seen her struggle with.
A concise web site of techniques, terminology and different species care. Yes the info is out there but its scattered around. For a young person with a full time job and in a new romantic relationship, she doesn't have time to hunt down multiple websites for the info she needs. I see most senior members recommending books but lets get real here, the younger generation doesn't want info in book for they want it online. I would love to see a website geared towards beginners with a TON of simple info and species care. I still struggle with timing of airlayers
Keep it simple for newbies. They don't know what a candle is or how to decandle a pine so of course they have no idea what to do with that new pine they just bought so they kill the poor thing. I see this in videos too, very little actual explanations in laymens terms.
Prices of pre bonsai material. DD has been looking for something decent that she can develop herself but everything is crazy expensive or hard to find. Keep in mind, for a youngster, $100 can be insanely expensive. The people who have decent material for good prices don't advertise or have a regular website to purchase from. You have to hunt them down on FB and only after someone gives you their name.
For sale material of airlayers or dug material for $50 or so would be perfect for newbies or for parents buying them for younger children. This could be added to the above mentioned website. I know there are lots of places you can buy beginner bonsai but most of them are crap and you know it.
Advertising, like those annoying little banners on top of this page but on websites younger people might be on. FB and youtube are great places to advertise, so are virtual pet websites. If you don't know what virtual pets are check out aywas.com or flightrising.com, these are filled with 20 year olds, a great target range.
Simpler youtube videos. don't just show a juniper getting pinched and wired, explain why you are pinching this and not that or why you are bending that branch down and not up and keep it professional. No one wants to hear your life story or about "this one time" and lay off the music, instructional videos need to be narrated, not a jam session.
Local bonsai workshops would be wonderful but I realize that can't be done everywhere. Many places do not have bonsai experts or schools but maybe a chain store would be willing to host some kind of event once a year. Say select home depot has a sale on prebonsai material along with a free cds/pamplets with basic info and a link to a website, said website would need to be the ones supplying the trees. of course.
Some kind of a kids day at larger bonsai shows. They could learn basic care for potted trees and buy reasonably priced trees in silly pots, spongebob or something like that. Have some contests for free trees, quizes would be good. Kids could get rewarded while learning proper care. Do a lot of local advertising for the bring a kid to the bonsai show and win a tree.
I think there is interest in bonsai but it's such a hard hobby to get into and fairly expensive. Make it easier to learn and cheaper to begin will go a long ways.