I used to help select candidates & perform interviews for programmer jobs at my last place (large European publisher), and heres my thoughts on the matter:
Profesional games development usually look for one of two things: Experience, or potential.
2 years writing games is good, any more is better.
However, the thing which really shows it is a good demo. It doesn't have to be a quake-beater by any means, but for mainstream console work you should have something using 3D, showing movement and decent graphics (not programmer art - there are plenty of free textures out there that can make a demo look halfway decent). It doesn't have to be a game! Console games are written by large teams, only the top few need to be able to put whole games together.
The other end of the spectrum would be Java oriented web games or mobile phone games. For these, a demo of a 2D game is more appropriate again with respectable graphics, effects, and preferably and good gameplay. This should be some kind of playable game. these are generally written by much smaller teams, and so demonstrating you can handle ALL the input/AI/collision/etc. is more important.
Things employers will be considering:
Given your History degree, are there other roles/aspects where it could be useful? What skills did a history degree give you (researching, thouroughness, working with other people, etc.) that can be applied to development? The fact you changed direction can be counted in your favour if you show it demeonstrates flexibility and a willingness to learn.
Age: this is an important concern because of salaries. The older you get, the more responsibilities you have (wife, mortgage, kids, etc.) and so the higher the salary they will expect to pay. Note that this is 'expect' - this sort of consideration will happen when they see your CV, before you even get a chance to talk to them.
Most places will consider hiring a beginner (on a low salary - but beware the expected expectations mentioned above) if they can show that they have the potential to be good, the willingness to learn, and the ability to fit in with teams. A well made demo or two can show you have the potential (& the necessary starting skill), but the other aspects are just as important to a potential employer.
Don't be concerned about companies 'ripping your demo off'. Firstly you shouldn't do anything too innovative (they may not get it, etc.). Secondly, after a few years writing professionally you will look back on your demo and think 'Flaming Heck - that was rubbish!'. And last but not least, almost all publishers will consign your demo to the bin after viewing it. Partly for legal reasons, but also because they are only interested in seeing what it shows you can do programmatically, and not what the content/idea actualy is (there is probably not a huge amount you could show that would be new to them).
Sorry for the long ramble, but I hope it helps
(PS: I know of only 3 testers who made it into actual development. one as an assistant designed, one artist, and one coder. They had a VERY hard time of it, needing nearly a year of pestering to get the chance after they first said that was what they wanted to do. Dev. tends to look down on testers, particularly those who 'think they can code too'. Sad but true.)