I'm not sure I'd call a lack of buying things now compared to in the past "maturity" but I think I understand what you mean: your tastes and approach to the hobby evolve to a point that gives you greatest satisifaction - and usually that is not acquiring new locos and layout stuff as much as being involved with what you have. However, a counterargument would be that reduced spending is a natural progression that will happen regardless: initially you have to spend a lot because you have no locos, rolling stock, or layout, and so you do, but gradually you acquire all of that so that then you have fun and don't need to spend as much.
But I think there is a point in what you say about where the enjoyment comes from: not just acquiring new locos, but from running and working on what you have. Here I think it takes a bit of time to discover what you like best. In my case I initially built a big trainyard with many sidings and over two dozen switches, etc., so I could leave up to a dozen made-up trains on the layout at one time and operate them in and out of the yeard, etc. That was removed two years ago because I didn't enjoy operating that at all. In its place is a lake with my moving ski boat (a fun, year long project) and a big town with lots of buildings to model. I keep only three or four trains on the layout at one time now (all that I can run at one time) but enjoy them, and the layout much more. That is the kind of evolution I am talking about.
As to spending. Initially I had a long list of locos I wanted to acqurte in O gauge, Over the past seven years I have filled in that list to where only a few remain to be bought - that will depend on when and if Lionel or MTH makes a premium level N&W J (next year, I have it on pre-order) or a ATSF 2900 Northern (who knows) or a Legacy Veranda Turbine, etc. so I don't expect to spend nearly as much in 2013 as in some past years.
Still, it is fun to lift a new loco out of the box, unwrap that plastic and liner from it, and runit for the first time . . .
