Although I don't have any personal experience in the field of recruiting I don't think that the training costs are reason enough not to hire someone beyond a certain age. Of course the company has to gain more money out of one pilot's workforce than that pilot actually costs - that's the way our economy is working. But this applies to any pilot independent of age.
On the long run the expenses for all the recurrent training sessions will be much higher than the cost of one single type rating course. The difference with respect to training costs between keeping a pilot for lets say ten years compared to 30 years is therefore negligible. If a company is telling you: "We would hire you but we can't afford the typerating." - how will they ever be able to afford all the necessary recurrents in the years to come? Probably such a company won't keep you anyway after your self-sponsored type rating has expired.
There must be more to the ever-lasting age discussion. Perhaps employers are afraid of "social disturbances" within their team if a middle-aged career changer starts all over at the very bottom of the seniority list? Maybe they consider you not tough/flexible/healthy enough to withstand that volatile business? Or they don't like the idea that one of their employees has yet another professional option outside their sphere of influence? I don't know - can someone with insight in HR possibly shed some light on this?