Buying Young Citizens is only beneficial to those who have a city big enough to train them and receive tax refund during training time. Citizen Training is a viable strategy, but you need to be focusing on Citizen Farming to see the ROI capabilities. Young Citizens are a great buy for cities that can support them, but it will negatively impact smaller cities that aren't equipped. The most efficient setup is going to be the Wind Turbine, Economist path that you can find here @ryosai/100x-windmill-economy-citizen-trainer