Adult onset ADHD
The short answer is that adults do not develop ADHD on their own. Several symptoms that cause impairment must be present in childhood in order to meet the criteria for an ADHD diagnosis. ADHD symptoms must be visible before the age of twelve. 2 This means that ADHD does not manifest itself in adulthood.
To put it another way, if you have ADHD as an adult, you probably had it as a child as well. As a result, a person's illness may not be diagnosed until later in life. If, on the other hand, you never had similar symptoms as a child, your current problems could be the result of something else, such as sadness, anxiety, or another mood disorder.
ADHD can be difficult to identify since symptoms vary so much from person to person, and it is diagnosed primarily through observation rather than more solid methods like blood tests or other physical markers.
It's possible that no one knew to look for ADHD when you were a child, and you've had it all along. As a person grows older, ADHD symptoms can emerge in a variety of ways. For example, hyperactivity in younger children may manifest as an inability to sit still, whereas adults may just appear restless.
Additionally, some people with ADHD find ways to cope that can mask their symptoms, such as fidgeting, using organizational supports, including lots of physical activity in their schedules, or consuming a lot of caffeine, which acts similarly (albeit to a lesser extent) to prescribed stimulant medications like Adderall (amphetamine and dextroamphetamine).
It's doubtful that ADHD is the problem if you suddenly get symptoms in adulthood that appear to be comparable to ADHD but that you've never had before. Talk to your doctor if you're worried about your memory, inattention, or other troubling symptoms. Depression, anxiety, sleep problems, and even menopause can all appear to be symptoms of ADHD in adults.
You'll likely feel better knowing what's truly going on once you've received a correct diagnosis, and you'll be able to focus your efforts on finding appropriate treatment alternatives.