All the lead didn't help. And the playing with mercury in school. Actually, there was lead involved there, too, as I remember my dad telling me about how they'd put a piece of lead in their cup of mercury to see that it floated.

Also had a science teacher clarify that it was mercury oxide that made playing with mercury so dangerous, with the anecdote of knowing someone who tried to commit suicide by drinking a large amount of mercury, but then only ended up with the shits (and probably a few less IQ points from the mercury oxide vapours he would have inhaled while drinking it).

Edit: oh, also the wars probably helped age people faster and no idea when this was filmed but he could have fought in either war and started acting after.