Have you looked back at the paperwork to see what was said?
Often people say nothing was said verbally about things like leases, but when they look at all the mass of paperwork, they see something mentioned inside it, such as buyers needing to be aware of various potential issues with leases.
The reality is that people often don't fully read or fully absorb what is a said. They get excited about their purchase and don't do due diligence in terms of checking things or even reading the information they are given. Sometimes thry read it and do t fully understand it....but don't enquire further. Solicitors are of course careful not to put themselves in a position to be culpable. Sometimes they do, but often when people look at their paperwork, the issue was mentioned.
What about in your paperwork Op? You are clearly hoping you might bring a negligence claim....but the starting point would have to be the paperwork. And as others have said, rightly there has to be a time limit for these things and your purchase was very many years ago now. To be honest, you haven't been 22 for along time (not that age will ever be acceptable as an excuse) and had many years to look at your lease again, plus there has been a lot in the press over the last years about leases. The fact you have only recently got to grips with the implications is pretty surprising to be honest.
But again, blame isn't really relevant or helpful at this point. Your stage for making a case has passed and blaming them or yourself won't achieve much now....better to focus on dealing with the issue and finding a way to extend the lease - look forward, not back.