Members at the party's Scottish conference in Dunfermline, Fife, passed a motion which also demanded "urgent" action from health services to make it easier for adults to be diagnosed with the condition.
Those with attention deficit hyperactivity disorder (ADHD) tend to suffer from either hyperactivity and impulsive behaviour, or can struggle with inattentiveness.
Sufferer Ross Stalker said ministers had an outdated and "obsolete" perception of the condition.
He said: "They seem to have reverted to an obsolete view of ADHD being a behavioural and a learning disorder mostly affecting children, but that's not what ADHD truly is, it's a cognitive disorder, it affects how the brain works in adults as well as children."
He added: "The Scottish Government needs a strategy that recognises what ADHD truly is, a lifelong cognitive disorder.
"So long as the perception of the typical ADHD sufferer is an unruly child, indeed an unruly boy, most adults with ADHD will continue to go undiagnosed."
Adult ADHD was dropped from the Scottish Government's latest mental health strategy, Mr Stalker said.
He added that those with the condition had been "let down by a system that has failed to act on the latest research which shows that the majority of people with ADHD right now are adults, and the previous perception ADHD is a childhood disorder most people grow out of is entirely wrong".
He recalled: "I can remember clearly at 27 years of age the first time I felt ADHD medication working for me It was during a meeting at party HQ in Edinburgh, someone was talking and talking and I realised I was taking in every word instead of missing half the conversation, nodding along and faking it as I normally would. I was really paying attention and for the first time in my life it didn't take an exhaustive amount of effort to do so."
A doctor told the conference he was only diagnosed after he had retired from the medical profession at the age of 65.
Dr Alan Rennie said: "As a child I was pretty wild, unruly, and almost sleepless. In primary and secondary school I was inattentive and not absorbing normal teaching."
He added: "At 65 I retired and eventually sought specialist advice, when I was diagnosed with ADHD. It was a great relief just knowing the diagnosis, a huge relief."