Nick, people have indeed gotten jobs at 40 and above however the detail is in the background. Perhaps they already had some instructional time or more importantly contacts (that really is the ace card).
Nevertheless yes you can become a pilot post 40 but as what airlines? Well still maybe especially if you have some good time under your belt both quality and quantity (turbine would sure help). Maybe turboprops or an advanced ratings instructor (once again you'd have to work your up through that chain). So yes it is possible but at what price and in the end to what extent?
For some who have an established profession/career and some financial security they can afford to take this option for others how are on the edge and strain their neck to stay above the water line it would be somewhat risky. Though that doesn't disclude them neither it just promotes the level of risk that's all. Factor in the state of the industry against the backdrop of the general economy's influence and you could find yourself qualified and met with a downturn at the wrong time. With a younger age brackett you could weather the storm so to speak and once the industry re-awakens in a few years 'get the last bus home' before you get too old.
However a lot of that commentary above is more anchored from yesteryear and with such dynamic changes brought to the industry by the LCCs I see many sacred cows being slayed. Couple with that the exploding aviation growth in India and China (where millions that used the train system religously can turn a few days journey across the country into a few hours) you may even have to work overseas but hey is that really bad?