As someone who works in media and marketing, there is a lot more going on than just disposable income. I think a lot of the conversation is circling around, but missing some key points. And yes, this largely applies to the US market, but since that is the primary market of WotC, that seems the place to focus on.
For toys, games, and entertainment media, teens (and more recently, tweens), have traditionally been the huge driving force (there are exceptions, I am painting with a large brush here). The secondary market is "Kidults," a fairly new market which is defined as age 12+ people who buy toys and related products for nostalgia, which is kind of a new category (think Beadle and Grimm, Funko Pops, etc). (
Adults are buying toys for themselves, and it's the biggest source of growth for the industry) This is why PG-13 super hero blockbusters are the target for movies, why traditional nerd brands are surging, etc.
Teens and tweens don't have high quantities of gross money, but they also don't have tons of standing expenses (groceries, bills, rent, etc). Almost every dollar they have is discretionary - and they have more money now than they ever have, historically. Purchasing power actually drops once they turn to adults (post-college), as 20-somethings take on life responsibilities they didn't have before. And older adults (30+) have traditionally been difficult to sell to; they are set in their ways, have pre-established brand loyalties, and are just much harder to crack into. Older adults also don't drive cultural zeitgeist. Teens/tweens are much more likely to become strong product evangelists, and to generate sales organically, and they aren't set in their purchasing habits or brand loyalties yet.
For households, a ton of purchasing decisions (especially around entertainment, toys, games, etc) are driven by kids (
Nearly 9 in 10 Parents Say Their Kids Influence Their Purchase Decisions - Marketing Charts). The king of this is Disney and their Princess lines of products, which skewed to marketing even younger than the tween audience. This was a market that did not exist 20-ish years ago, and completely changed how Disney did movies, planned their parks, produced live shows, and marketed to mom and dad.
From a marketing perspective, you don't ignore any demographics, but the easiest way to get long-term people into your brand is to catch people when they are young. There is less work involved, and if you can get them into your brand then you have customers for life.
I'd say that WotC is doing a multi-tiered approach. The movie is definitely teen friendly while also nostalgia Kidult targeted. 5e is a nostalgia throwback of a system that cleans up for modern sensibilities. 1DND is a continuation of that, making changes that clean up the system but still play in a nostalgic way (for all those people who came on board during 5e). They produce books and tools that can be bought by anyone since the price points are ~$25-$30 (dndbeyond, Amazon), but also have partnerships and lifestyle products that speak to higher end nostalgia buyers. They are focused on building a brand, not just products, which will tie into a lot of the identity that kids seek from their media and toys.
Focusing on just older players is a good way to kill the brand long-term. For WotC to grow, they have to build the core audience through their core product. But they also can't ignore their older players, because there is a lot of high-dollar disposable income there. And they need different products (high end lifestyle and nostalgia products) to fully capture those dollars; their primary books are never going to be the way to suck that extra money out.
Right now, they seem very focused on 1DND, which is core product. I expect that once we get through the launch of that, they will try to expand their scope more, but I also expect we will see a lot of it through licensing and not internal development. We've seen the first of that with stuff like Beadle and Grimm, which is very smart licensing. WotC is currently very good at producing 5e products, but they don't have a lot of wide reach into further fields. Branching too far too quickly is a great way to hurt a company, while licensing limits risk but grows brand and profits.