Managing your expenses means understanding what you spend your money on, making wise spending decisions and maintaining a balance called “Living Within your Means”. An budget plan or an annual financial plan will allow you to do plan, track and manage your expenses for the current year. Once you have that you can then use that to project your expenses over future years and of course make adjustments based on what you know or presume about your future. That goes into your lifelong financial plan.
But how would I expect my expenses to change as a result of life events such as getting married, having children, changing careers or retiring. That depends on the event of course and only you can make that judgement. For example, you may plan that when you retire your lifestyle and spending habits will continue as they always have. That is a good assumption and starting point. Then factor in other things such as that maybe the children have left home and the expense associated with them will reduce. Or perhaps you expect quite the opposite, you presume a possible need to support adult children at home or elsewhere.
The point is managing your expenses is not just for now, it is for life, and you need a financial plan to do that.