I knew a fellah who had a cleft in his soft palate that was never fixed as a child. It wasn't something you could see unless he showed you. It affected his manner of speech in a way that might be thought of as a lisp. I guess this would still count as curable because there is surgery for this, but without it, the speech pathology is being caused by the connection of the mouth with the nasal cavity. It's structural. This is not something that speech therapy could fix.
Some lisps are due to anatomical defects. Fix the defect you fix the lisp. Not all cleft palate defects are 100% repairable so there may be some anatomical lisps that are not correctable. However, not all lisps are anatomical. Some are functional meaning the person has not or cannot learn to make a certain sound. Just as I cannot roll my r's when I speak Spanish, some problems can be helped with speech therapists and some not. It is important to address these issues early as there is a window period we learn to make certain sounds. That's why people have no trouble learning their own language sounds but as adults cannot always learn the sounds of another language. I can't roll my r's but many Spanish speakers can't pronounce 'girl'. http://speech-language-therapy.com/index.php?view=article&id=86:lisp
Mmmm it just occurred to me that if the lips was still noticeable at 18yo ... it would be unlikely to have completely disappeared at 39yo ! ..... I think that'll do it
I don't know much about lisps but, when I was sixteen I used to purposefully speak with a lisp when I got on the bus so that I could get away with a child fare instead of paying full price.