Have done a bit of googling, and have found things like this:
Over 65's travel insurance
If you are aged between 65 and 79 Right Cover can offer insurance to Europe, Australia & NZ, and Worldwide excluding the Americas.
So... it would appear that there is indeed some issue regarding travel to the USA.
There may well be policies available that don't have this exclusion (I haven't looked very hard) but it looks like they will be a bit more difficult to find. My mother-in-law travelled to the USA at 70 (a few years ago) and was able to get insurance, but I don't know what sort of conditions were attached to it.