My buddies and I picked it up in middle school, and we still play as 30 year olds, now. We used to scrape and save for models, use bartertown for good deals, and basically fill our Christmas lists with model requests. We also proxied like crazy (played in my basement on some plywood sheets atop a pool table). In fact, we probably had a heck of a lot more pure fun from those games even if our models were ugly, the terrain was slapdash, and the rules were botched. It can be done, but the price point is very, very hard for younger folks, especially ones that want to go to the mall, buy video games, pay for gas/insurance, etc.
I am a high school teacher now, and I run a gaming club for my students. Some picked up 40k last year, and I made sure to explain how financially demanding the game can be. Fortunately, they have something I didn't, which is a gaming store that has nice terrain and a free play area, so they don't have to shell out for scenery and what not. I've found that the secondary items (codexes, data cards, paints, brushed, dice, etc.) is a bit of a hurdle. $40 for a codex is really harsh when all you want to do is play a game. When you have little $$$ to begin with, buying models, since they can actually be used and are more fun, is the priority for my students. Definitely not an adult-only hobby, but also not a hobby for your average younger person.