Kaola
I don't think it's as simple as old= shorter life, young= longer life.
You have to consider the possible impact on a child(ren) that watching a terminally ill parent could have. This could be truly devastating for them, compared with- and I don't mean to sound glib and mean this generally, just not for the OP- a quick and fairly dignified exit.
The strain on a child as they approach their teenage years with all the hormonal ups and downs and GCSEs etc could be far worse than losing a parent at a younger age.
One of my DCs friends lost their mum to cancer when they were 13- but they had already lived with her cancer from aged 4, then 5 years remission, then it returned.
It's not morally right IMO to suggest that someone who is ill, possibly in pain and suffering great indignity through illness, should hang on to life at any cost so they can have more time with their loved ones- or more, as you seem to be saying- their loved ones can have more time with them.
I feel terribly sorry for the OP and her family, but really it's her DH's choice ( with some consideration for his wife's views) that count.
It's not clear from the OP's posts what the prognosis is- whether it is in fact a terminal case, or maybe terminal without surgery, or maybe even remission with full surgery and further treatment.
As rabbit says, I doubt if anyone with such a prognosis would have ' 5 great years' because there would be a slow decline, so it might be a case of 2 good years then 3 pretty awful years.
Op- don't know if you are still reading but you said your DH was having some screening soon, so hope the results give you some hope.