Lego sets are very complex toys, but the TRUTH is, a child that is capable of putting together a set marked 5+ is ALSO capable of putting together a set marked 17+. He/she will just need help.
My husband is a HUGE lego fan, especially the Star Wars stuff. He bought that giant Death Star set...and we've been working on it for MONTHS, and it's still not done. He and I AND our son all work on it during the weekends for about an hour at a time. It's a collaborative effort. I honestly don't know how ONE person could put that thing together...regardless of age.
Our son just turned 5, and he's been playing with "small legos" since he turned 4. As I mentioned, he practically put the Beach House (an 8+ set) together independently. He has Autism, and his brain is just wired with that spatial skill set that makes him REALLY good at doing Lego sets. It has nothing to do with "maturity", because he does get frustrated, but he also is a perfectionist, and his frustration stems more from being overly forceful with the bricks sometimes, and the pieces breaking apart mid-build. When he "messes up", he feels he has to start over. So, there's a lot of convincing him that he can just fix it and keep going where he left off, rather than destroy the WHOLE thing and start from Step 1 again...
Lego sets are a wonderful teaching tool. We use the Legos as a way to teach patience, perseverence, fine motor skills, and how to follow directions. On their own, the legos are great for inspiring creativity and imaginative play. I think they are one of the best toys ever invented.
In my humble opinion, the sets shouldn't have age markers on them. The kids should just get sets that interest them. And, parents should be prepared to help them if needed.