I've heard of it a couple of times but from damage, not age, and usually causes trans fluid to contaminate the radiator as well. Fair bit of work to clean it all up but haven't heard of any long-lasting issues once it's been fixed. I don't recall the costs but labour will be the biggest hit.
It's certainly a good case to argue in favour of separating the transmission cooler. Mine's the same age as yours (about 147,000km on her now) but I have just had my radiator rebuilt so I don't think it's going to be an immediate drama for me. I still might go get myself a separate cooler and whack it in just to be safe.