I can only verify anecdotally but I was only properly diagnosed at 31 and before that my brain was running at 110% all hours of the day. No coffee or other stimulants. Had a lot of trouble sleeping, bad anxiety, depression, terrible emotional stability, difficulty concentrating in school (high school, college twice). After getting some medication, and some practices in place, it is a night and day difference.
If anyone feels like you have difficulty managing your brain, in a sense, make sure you talk to a therapist. While ADHD isn't always the cause here there are a lot of symptoms that coincide with it that can make it difficult to pinpoint and require you to run the gauntlet of medication(or therapy sessions). I had to go through anti-depressants and anti-anxiety before reaching ADHD medication that seems to curve all the symptoms at once.
Understandably, a lot of people are skeptical since some doctors and most pharmaceutical companies aren't known for being forthright but it does exist to a certain degree so don't let it ruin your life if you feel like something is wrong. The symptoms can begin to stack over time and get worse.