Some are, some aren't. Some of the young adult books deal with things like high school cliques - and these would bore a 20-year old to death. But some young adult books are well written and have a good following among 20 and 30-year old readers.
Just as an example, go to smashwords.com or amazon.com and look at The Long Way Home, by Florida Town. This is a story with lots of action and adventure, plus discovery and survival - and a few surprises. It's a story that can be read and enjoyed by any member of the family - young adults, regular adults or old adults.
So young adult isn't always a good indicator of the age-level, but rather the content level of the book. The Long Way Home doesn't have graphic sex scenes in it, so it can be marketed as a young adult book - but it can also be marketed as an adult book because some adults don't enjoy a book that is heavily saturated with sex.