Well, until he denies being on any meds, it's as good an explanation as any, and extends the common courtesy of giving him the benefit of the doubt.
But yes, as a general rule of thumb, increasing age is directly proportional to increasing incidence of 'prickly' behaviour. In the final year of his life, my dad was accusing me and my two sisters of stealing from him. He left everything to his 'carer'.
I was surprised to learn later that this kinda behaviour is by no means unusual.