Mosfet life expectancy?
Generally, the parameters specified in the datasheet are for a functionally infinite lifespan, e.g. MTBF of more than 1 million hours. Life expectancy is not statistical anymore at that point, as you would need to have many thousands of products in circulation to get a statistically significant amount of failures. Most failures will just be 'luck of the draw' - not something you can design for.

If you're going over spec or if you're doing especially destructive things (e.g. very strong current pulses, high frequency driving, etc.), you're entering the domain of 'designable' lifetime. This, however, depends very heavily on the internal structure of a FET and the specific failure mode you are designing against. For instance, bond wire burnout is something you can design for in extremely high frequency dc/dc converters (where the gate bond wires are loaded particularly heavily).

As said before, these parameters aren't in public datasheets and it's likely the manufacturer doesn't even have consistent data on them.