It is highly fact sensitive as with any use of force law. If a homeowner in a crime ridden area saw two people breaking into his property and running towards him for example, he would easily be able to claim self defence. Especially as a 60 year old.
If instead, he saw them at the tree picking oranges, that self defence argument wouldn't be very strong.
Something like this:
Would likely be completely lawful to deter someone stealing your property
You just need to be able to be better than the average female cop in the US and not confuse your less than lethal firearm with the lethal one.