For what it is worth, I do think that the issue may also be down to space as well as cost. Here goes...
I am 27 and own a house with my wife. It is a new build 4 bedroom house. You could say that we are in the fortunate position of both having good jobs and without children are able to spend disposable income on our hobbies. I do feel that 150+ on a loco is pricy but can afford a few a year.
However, I have limited space for a layout, modern houses do not lend themselves well to sheds in their small gardens, and as many modellers can attest to, having space away from family life is difficult. This is compounded by the fact that I don't want to build up a layout in a room and then a few years down the line have to remove it for a new child.
Of course when children have left home the space is available again, but then I would be older.
As I noted above however, I am in a very fortunate position and most people my age are not in such a position to own their own home regardless of size, and so that may also hinder their ability to model /find space for one.
That said, the concept of a micro layout is now very appealing - it is smaller so obviously can take up much less space, and only needs limited stock which keeps space down. Perhaps the days of large layouts are numbered...