40k's got something for most everyone, really. I wouldn't call it an adult-only game.
As almost everyone has brought up though, cost is a thing. Myself, I got into it ages back during 2nd edition via small armies and multiple proxy models. The models were also a lot cheaper then. Since those days, the models have become much pricier (cooler, but very expensive). That can price younger people out. Heck, it can also price out multiple adults in less-than-ideal financial situations (although you can get by with a pretty limited monthly investment if you're in no rush).
Speaking of change with time, I think models are harder to assemble now. We get great customization, but it is trickier. I remember trying to get various metal parts glued as a kid without knowing about pinning, and it was tough. I can envision (some) younger people being frustrated with the models.
I can also envision frustration with painting. I /love/ painting, and think you really get what you put into it. Now, I'm not stodgy enough I'd refuse to play someone with a just-primed army. But if you leave your army in that shape I think maybe you're missing out. Some kids are probably better painters than me (i'm definitely not winning any awards...) but it may be hard for many others.
The rules. Easier to learn than when I started in 2nd ed? Maybe? Probably. It doesn't really matter though. There's still a lot, and knowing them is critical. I'm totally cool playing someone who doesn't paint, and get that painting takes both time/money/skill. But playing someone who doesn't know the rules very well is a huge turn off. I won't do it (outside of new players, of course. I get that it takes some time to pick up. But make a serious effort to pick it up. Don't just keep showing up not knowing things. Learn at least one thing cold every time you play.)
Other games are probably more accessible, and easier to learn than 40k. Those probably also require less time, and almost certainly require less money (especially if we open this up to video games/pc games, etc).
Last issue: many of the above things don't bar kids at all, but constitute obstacles for them of various heights. This leads to many more adults being present in stores (probably). If I was a mother of a young-ish kid, I'd honestly probably feel a bit overprotective and would worry about sending him to play games with random men when I wasn't there. Personally speaking, my mother never got on board 40k when I was a child. There were several reasons, but that was definitely one of them, so I'm not making it up. (And yes, I do know other parents are much more supportive and even introduce their kids to the hobby... just saying though. It can swing both ways).