As you probably have an idea, there probably is no solid answer for this. Decades ago, they said there was an age cut off of 40, then it became 50, then the age limit sort of just went away. The reason is because age is just a number, and how well you will adjust or be at risk for complications has more to do with your general health and state of mind than your age.
Sure, the older you are, the more likely you are to have issues with healing, sphincter weakness, all that stuff. But, it is on a case by case basis.
The fact that you are waffling on this now that you have your colon out and living a better life, even if it is with an ileostomy you didn't bargain for, probably means you might have other issues you worry about.
Unless your rectum is behaving very nicely, you probably will have to address its removal sooner or later. Cancer probably isn't a huge weight on your mind, since your diagnosis was pretty recent (cancer risk doesn't generally even begin to go up until about 8 years after diagnosis). But, that goes out the window if you had active UC for a long time that went undiagnosed. If you do have to deal with that rectum, that is another surgery, and not a simple one either.
You can talk this over with your surgeon (but I bet he will make it seem like sunshine and rainbows). But, any good surgeon should be able to be realistic about YOUR case. Maybe have a visit with your GI and let him talk about your options.
The main thing is to be sure you are 100% behind whatever decision you make, because YOU are the one who will be living with it. I know there are many people who have the j-pouch well into the 6th and 7th decades of life, some even later (but fewer for sure).
There are no right or wrong choices in general, except how they apply to you.
Good luck!
Jan