Well, my father will be 78 this year, and he still hits his 2-handed backhand. In fact, his left shoulder is so shot that he serves right handed, then switches hands (he's a lefty). But he still had no problem hitting that 2 hander.
But, I do agree that most older players hit a 1-handed backhand. In fact, I was in Naples, FL over X-mas, and other than my father, I cannot recall seeing anyone over the age of 60 hit two hands.
"Nobody beats Vitas Gerulaitis 17 times in a row" -- Vitas Gerulaitis, after beating Jimmy Connors on his 17th try.