Really it is a genetic disorder,my husband his it (not too badly) and my daughter has it.She has just been diagnosed as a teenager. It IS a physical problem to do with chemicals and connections in the brain (without getting too scientific)
You cant 'Get it' in your 20s, you are born with it but if you have it mildly you may not show it until later. My daughter,who is very intelligent, wasnt diagnosed until 14 as she sailed through primary school but didnt really have difficulties until she had 15 subjects to cope with and showed very poor organisational ability, and concentration problems.
There are diagnostic tests and a Pscycologist can do it, you can also look it up on the net and do it yourself.
We have chosen the drug-free approach so far.Omega-3 is supposed to help but my daughter would disagree.
Good luck.