One of the diagnostic criteria of ADHD is that it’s life long, which means it can’t be acquired. However, it’s possible to acquire symptoms that are similar to ADHD, but then it’s probably something else.

As others have said, it's a life long thing, but a lot of the patterns can be brought on in people who don't have it with consistent sleep deprivation and a state of tiredness and exhaustion. If you're starting to relate more and more to ADHD memes well into adulthood, try to get a few nights of good sleep!

I mean, you could get a psychosis which results in executive dysfunction. I have a friend who had a psychosis which changed him quite a bit.

The extensive research on ADHD has shown that it is a developmental disorder of the prefrontal cortex, causing dopamine signaling pathways to act differently than expected in neurotypical brains. Because it has to do fundamentally with how the structures form as the brain develops, it's very unlikely that one can acquire it later in life (neuroplasticity is a thing but I've not read anything to suggest that it could alter the brain in such a fundamental way).

Some possibilities, that others have mentioned:

• High-masking ADHD. It's possible for one to unknowingly have ADHD their whole life but succeed in making their symptoms so well that it is not externally apparent and internally thinking that their experience is typical.
• Chronic stress. Psychological stress is horrible for the brain and body (there was a great Nova documentary on it called "Stress: The Silent Killer"). Some of the symptoms of hitting breaking points and burning out due to chronic stress are somewhat similar to ADHD.
• Other psychological disorders. Sometimes anxiety and depression (both frequently comorbid with ADHD - YAY!) can cause attention and executive function related problems. Treatment can help to alleviate and those who also have ADHD, make it easier to focus on things (like working on their ADHD).
• Hormone imbalance. This is actually an interesting one that I got to learn about first-hand. There are receptors in the brain for both androgens (ie testosterone) and estrogens (ie estradiol). Men with hypogonadism (testes not producing enough testosterone, which, contrary to what the name might suggest, often results in swelling) have been documented to have symptoms similar to ADHD, anxiety, and depression that were alleviated with TRT. Slight deviations in hormone levels can have a massive impact on people who are sensitive to them.

ADHD is contagious, like AIDS

/s

Unfortunately, some people still believe this is true, just like some people believe AIDS is contagious by touch: https://www.npr.org/sections/13.7/2015/08/17/432541935/belief-that-mental-illness-can-be-contagious-contributes-to-isolation