ADHD is understood to be a brain-based, developmental disorder present from childhood. Usually adults who have clear signs of ADHD can find a history of attention, impulsive, or hyperactive problems in their childhood.
Some adults who have a milder case of ADHD don't "run into the wall" until they get a job or other life circumstance that finally overwhelms their coping strategies. If your parents and siblings are good historians and don't recall you having any issues with attention, hyperactivity, or impulsiveness, then other issues like anxiety, depression, or a really stressful and complicated lifestyle might be the issue.
These days, most people are expected to keep a lot on their plate and stay organized in a chaotic world. If you are only in adulthood running into problems, then it may be time to talk with a psychologist about your personal symptom profile and history to see if ADHD, another diagnosis, or "just life" is right for you.
If you are feeling overwhelmed lately, even if you don't qualify for a formal diagnosis, having counseling to address your stress (and likely disorganization) can be really helpful. Adult support groups for folks with ADHD can also help normalize your experience. CHADD is a group for folks with ADHD that publishes a list of local groups along with lots of helpful ideas to manage ADHD and its core symptoms.