Rather than lecture my 15 year old daughter, I use everyday experiences as opportunities to discuss the time value of money, the monetary value of time, being aware of costs, getting the best value for our $, sitting on a prospective purchase for a while before making the buy.
Everyday we have at least half a dozen opportunities for such discussions: e.g. 1) dad can I have $ for school lunch? 2) Dad, i need $ for book covers, 3) dad can you drive me to the store?, 4) discussions on the value of her getting a job this year, 5) why equity markets are tanking today, 6) why she needs to clean/care for item X (buy it for life).
Abstract discussions (#5) dont engage her well. But real life impact for her (#1-#4, #6) get a real engaging discussion going. Shes neither resistant nor "on board" with frugality. Little by little she comes to learn and understand. The headwinds of blind consumer consumption blow, and blow hard they do. Our discussions will hopefully temper such winds.
Yesterday, I asked her why she wanted more money which of course resulted in the expected response. But I then queried her on why she wanted those "things." Which got her to start the necessary critical thought. It may take a decade or two for her to arrive at honest answers.