There is an “Age assumed dead” setting. It’s on Administration > Family trees > your family tree > Privacy.
But you will almost certainly find that is already set. It is enabled by default, at 120 years.
The primary purpose of that setting is for privacy calculations – is a person dead or alive, and therefore can they be displayed publicly or not.
For the statistics you need to actually have a “confirmed” death for each individual. It would be totally impractical to calculate it for every person who “might” be dead.
When you create an individual it is a good idea to add a death with the check box at the top of the edit screen ticked, even if you don’t know when or where they died (assuming they are likely to have died.
But importantly, in addition, it’s a good idea to use the Batch update “Add missing death records” tool regularly. I try to run mine once every month or two. (Administration > Tools > Batch update > Add missing death records)
This uses the “Age assumed dead” setting from above to tick the “Death” box for any person deemed “dead”. This achieves two things:
1 – it tidies up the very statistics you questioned, and
2 – it speeds up the isdead() calculation for privacy and display of “living vs dead individuals wherever that is necessary.