Does diabetes (Type 1 or Type 2) shorten person's lifespan?

It can, especially if out of control which means risk of coma, stroke, and so on.   However, my Type 2 D, hasn't always been as under control as my D. Nurse would like and I'm now approaching 80.   My sister on the other hand, died on her 68th birthday but then she smoked, had asthma, arthritis - you name it.   Her type 2 D, was both tablet and insulin 'controlled'.

I believe that genetics plays a bigger part in anybody's life-span but both my parents didn't live that long (neither smoked but mum had diabetes) although my paternal grandfather lived to be over 100.   Go figure.

People with type 1 diabetes, on average, have shorter life expectancy by about 20 years. People with type 2 diabetes, on average, have shorter life expectancy by about 10 years.

Only drastically.   With both come compromised immune systems.   With type II,  dementia is what follows.   The cause of type II also causes heart disease,  liver disease, hypothyroidism, PCOS and many skin disorders. 

That'll do it. 

It really isn't the actual diabetes that is the problem.

It is the complications that happen because of poorly managed diabetes that is the real problem.

I know of a type 1 diabetic that died at the age of 40 due to diabetic complications.  I know of another type 1 diabetic that died at the age of 55 due to pneumonia and kidney failure.  (his body was shutting down and his immune system was very low - he could no longer fight off illness - it was NOT actually COVID - it was just his body failing.)

Of the type 2 diabetics that I know - none of them have died yet and at least one of them is 71 and still fairly healthy.  I know several type 2 diabetics that are in their 60's and still in fairly good health too.

With type 1 diabetics - they are usually diagnosed during childhood.  A type 1 diabetic has usually had diabetes for 30 years by the time they reach the age of 40.  BUT - a type 2 diabetic USUALLY doesn't develop diabetes until sometime AFTER the age of 30, so by the time they have had diabetes for 30 years - they are usually in their 60's or even older.  The longer you live with the condition, the longer it has had time to do slow damage anytime there has been issues with controlling sugar levels.  This means a type 1 diabetic is usually going to have more diabetic complications at a younger age compared to someone who developed type 2 diabetes later in life. 

Basically - people with type 1 diabetes usually deal with the condition for many more years than a type 2 diabetic would.  If someone was to place a magic number on life span and say that a person can live with diabetes for 50 years - then a person developing type 1 diabetes at the age of 10 would have a life span of about 60 years old but a person developing type 2 at the age of 30 would still have a life span to about 80 years old.  I am not trying to say that the life span after diabetes is actually 50 years - I am just making a point that if it is possible to deal with diabetes and stay healthy for a certain time frame - then that time frame is starting at a younger point for type 1 when compared to type 2 which means that type 1 diabetics will usually start hitting health issues at a younger age.

How much it shortens each individual person's life really depends on how well that person has been able to manage their condition.  Good management means good health.  Poor management means poor health and a greater chance of dying younger.

It cuts it by about five years on average.

It can, but not necessarily.  It depends on (1) how well you control your blood sugar and (2) luck.