I am 27 and work as a GUI-dev. My co GUI-devs are 26 and 32 years old.
But the back-end developers are 38, 43 and 56 years old.
The management in here consists of engineers, too. They are all >40 years old.
Most tech-people I know either got in to back-end development or management when they got older.
If I had to guess why it is, I'd say that back-end technologies naturally evolve slower. Yes, we are in the middle of a Big Data boom, but even then, the rate at which Hadoop changes is nowhere near Ruby on Rails, to cite two popular technologies.
I think it is this way because back-end development requires more expertise than GUI development. Steady more experienced developers are better suited than new hires.
Slow = bad
Fast = good
They probably think a good GUI is a question of taste and can't be measured and they don't want depend on "luck"
In either area (frontend / backend) the complexity depends entirely on the problem that you are trying to solve and I am sure there are plenty of FE problems that require a high level of expertise to solve elegantly.
The back-end in the last five years or so has definitely had a lot of change in data stores, emphasis on RESTful API exposure to front-end and other back-end services, web server choice and style (WSGI, PSGI, FCGI, etc).
The biggest difference is that a front-end developer has to worry about the performance of a single client using a single state machine. The back-end guy has to deal with making a site fast and reliable while supporting 100s or 1000s of those clients concurrently.
Crash the browser every 100K views and someone reloads. Crash the back-end every 100K views and someone gets fired.
In the Dark Past, this last line of defense was in stored procedures, but we -- 30-something greybeards :) -- know what a mess that was. There are plenty of companies who are still tied to an ancient DBMS due to that 10kLOC stored procedure that looked like a good idea at the time.
Also, the back-end stuff tends to be a bit less sexy, but also less fault-tolerant. Management sees the front end; the back end is basically magic. So it makes sense that you have Gandalf back there with his beard and recumbent bike banging out magical koans to keep the database alive, while young bucks guard the front gate of the castle.
I worked for close to 2 years on a new backend for the insurance company I work for. A new, street-smart guy started getting cozy with management (instead of doing his assigned work), went behind the back of the IT manager and convinced another department to give him some money to build a colourful new frontend (which basically called the new backend and displayed the results, the money went into design and showed), and the company ended up firing the (pretty clueless) IT manager and putting the new guy in charge, over the guys that worked on the backend that made everything possible (both our fault and the clueless IT managers' that this happened, but it still sucks).
Work, as I have become older, is more and more just a business proposition.
Plus it puts me further away from those guys that think HTML/CSS is programming. :)