In general, car prices have gone up. More spent on emissions, crash and other regulation R&D. More materials. More expensive labour. A weak pound. Plentiful low-interest debt. Leasing. Longer warranties. The bottom of the market rising as old cars fail MOTs or have expensive VED.
Specifically for the Yaris, it's a high cost to practicality ratio and is near the top of its size segment for price. If you need to seat 3 full size humans then the car won't work for you and there are similarly priced cars that will. If you don't need to, there are cheaper cars that are very critically acclaimed.
Conversely, spending power has generally gone down for younger people. And I don't just mean 18-year-olds, I mean all the way up to people at an age where they are thinking about or living the reality of carting kids about. It's easy to reach an age where you can no longer contemplate dailying an effective 2-seater before you can expect to afford this car.
The appeal of cars for this generation has decreased. It's not cool to admit you like cars in a world that understands global warming to the extent we do and it's not a necessity in a large city, where young people are more likely to live. For those who still like cars, I would guess they're more likely to care about something German.
At the same time, the appeal of the car is weighted towards older people. As I said earlier, there are more practical, higher-horsepower cars for a similar price. There is some other appeal that makes us want to buy this car. For a lot of us/you/car reviewers, it's the appeal of the idea of a homologation special(ish). That's going to mean a lot more to people who were around to buy the last set of homologation specials or who were around to watch rallying at its peak.
So I'm not surprised it's mostly people older than the typical hot-hatch stereotype.
I’m just the right side of 50. 
