Hi, the answer to this is complex. It also very much depends on how well diabetes is controlled. If a1c is asst our close to non diabetic levels life expectancy, assuming no other conditions / familial problems, could be similar to people without diabetes. If blood sugared are chronically high then the likelihood of complications and shorter lifespan / poorer quality of life rises dramatically.
I remember reading that it's roughly 11 years shorter than normal, but it seems like a lot of things would play a role, such as how longer the person had had it, blood sugar control, etc. I also read about a man who had been type 1 diabetic since he was young who was 81, but he tested his blood sugar like 11 times a day. Back around 4 or 5 decades ago I think type 1 diabetics generally lived to be about 50, so as technology for managing blood sugars has improved so has life expectancy and things can only get better.