A lot of teens work in the US, to many IMO. That being said, mine didn’t have jobs (mostly, my youngest got a job as a research at the State University by accident basically) in HS. Nor did we give them an allowance. We did however basically buy them anything they wanted (except video games).
My point is there are millions of teens (in the US alone) and a lot of them have access to more disposable income (through numerous sources) than one would think. D&D really only needs to capture a faction of that.
And the most important thing is that those teens may become “whales” in the future. I spend a lot more on D&D now then when I was a teen ( even if it was my parents money), but I wouldn’t have spent any of it if I wasn’t a D&D player when I was a teen.