Certain abilities that normal people mature in are stuck at the level of a fifteen-year-old for me and will be perpetually. Some of them are:
Ability to "feel" the nearness of the future, and plan and act accordingly
Ability to choose to do one task over another, and particularly an uninteresting task over an interesting one
Ability to stop an interesting task to change to another important task
Ability to stay on a single topic in thoughts and conversations
Many people with ADHD also have the emotional regulation of a ~15 year old, perpetually.
Most people can approximately remember how difficult those different abilities were when they were fifteen so there can be some level of empathy. When you were fifteen, university most likely "felt" very far away, and preparing for it "felt" not very important compared to your day-to-day. In hindsight, 15 is not far away from 18 and starting university, and there are many things you can do at 15 to make entering a good university easier.