Age gaps and death gaps
As you may have noticed, relationships often have age gaps. This is fine, but it does imply that the younger partner is likely to outlive the older partner. Further compounding this problem is that the older partner usually is the man, and men tend to die younger than women to begin with.
So here is a calculator to show the probability distribution of the number of years between the deaths of two partners, based on age and gender. Of course there are many other factors at play besides those two, but this should give you a rough estimate.
I know this is kind of stupid because you can just take the actual age gap and add/subtract a couple years to account for the gender difference. But it's fun to see the probability distributions.