ADHD is commonly referred to as an executive dysfunction - it also effects non-executive tasks and time management in general but, coming into adulthood, you'll truly understand why it's a recognized disability and stare in shocked horror at ADHD havers of prior generations.
It was really unfortunate in school if you failed to complete an assignment and had to repeat a course or a grade but if you can't manage to pay rent or utility bills (even when you have the money to) you're fucked. It can also be difficult to keep jobs if you struggle to produce consistent output potentially depriving you of the money to pay those bills.
When you were a kid you (hopefully) would always have a roof over your head and food to eat - being an adult means losing that guaranteed safety net. Even if we've had years to practice coping methods the price of failure is extremely steep.
It's also for this reason that I've encouraged everyone I know in the states with ADHD to emigrate - the social safety nets in America effectively don't exist for us as they have qualifications and constant high amounts of effort to maintain enrollment. If you get fucked once in America you're going to be in a bad place.