Environmental: This is easy. The Earth could flourish as humans were dramatically lowered in numbers. The human population would be 49.5% smaller immediately. That would mean far less global emissions, far less consumption, far less environmental hazards, and more unoccupied space for nature to rewild. Oh my goodness, this is simply positive in every way, shape, and form. However, it could be a bit less lovely!
Social: If humans can only live to the age of 30 and know that this is a reality, they may become massively depressed long before then and do two things. Firstly, they may give up on life and become hermits. Without a long life ahead of them, the purpose for life would seem to vanish. Secondly, they may consume much more and become complete hedonists, not caring about consequences (because who cares?), and generally being total rotters! A significant breakdown in social values would take place as humans lost a powerful reason to be model citizens and pay taxes – old age.
Economic: If humans failed to live past 30, the tax income would disappear, too. However, a huge amount of tax goes toward paying for healthcare, so that might not be a negative effect. Any wealth held by those over 30 would be passed on or released back into the economy. Wages and salaries would sky-rocket and the quality of life would also increase rapidly. Cash and money would flood the world for the under 30s. Savings for a retirement or a rainy day would become quite pointless and the moment would become the focus. For those who continue to contribute to society, life would become a constant cycle of consumption of entertainment, cuisine, and knowledge. With a time limit, people would not sleep in and become much more efficient and productive. Or, they’d give up and do nothing all day.
Political: Brilliant! 25 year old presidents and ministers. No experience, some may cry, but who cares! Politics is traditionally dominated by people over the age of 40. Those over 40 tend to have enough money, and advantages (and back pain), and don’t want to alter that. Younger people tend to not have much, so it’s possible that younger politicians could enact lots of dramatic change. Or, just do nothing all day.