] Was the average adult life span in Regency England somewhere in the early 50s? I can't find this anywhere. It has to do with parent-grown child relationships in JA.

Be careful about concluding anything from averages. You are right that there was a low average life expectancy, but the stats are affected (a) by the high infant mortality rate, and (b) by the high mortality rate among the poor. Since Mr Bennet is neither an infant nor poor, he could have very many decades of life ahead of him.