Just as have said , unless the person doesn’t have much responsibility, but even if you are staying with your parents that doesn’t mean one will have lesser responsibility, apart from not paying rent and buying food stuff which I believe most of the time u will still contribute one or two to the family, there are little expense you must be involved such as data, transportation, probably light bills and other stuff.
It’s good to invest aggressively but I think it should be on a calculative risk , if the investor which to invest monthly, that means with his income , emergency fund will be kept , he will remove some money for his personal need then discretionary income is what he can use for his investment in other not to sell his hodi , what we are talking about it’s long term , so one need to be prepared because this requires patience if not he may want to sell when he run out of cash , so investing with half of his income is not really good.
Your comment is very vague. Putting aside some money for personal expenses is not called discretionary income. Discretionary income is the amount of money left over after deducting all expenses from your fixed income. If you put aside some money from your fixed income, then you are doing something wrong. You can keep some money from your discretionary income for emergency fund and you can invest. If you do not have an emergency fund, then you can invest and create an emergency fund at the same time.
You can buy with your discretionary income using DCA method. How much of your discretionary income you invest is entirely up to you. But I think if you invest 5% to 25% of your discretionary income, it will be much better for you. You can buy aggressively during the downturn with the remaining money you have.