My dad is 61 years old and was diagnosis with Alzheimer. He still recognises people, but is becoming more and more apathetic as time goes by. He's been getting quieter, losing vocabulary, and is becoming more dependent.
There are days when he seems "fine" and you can talk to him and he accepts doing things, other days he doesn't care about anything, says he'd better of dead and it's really difficult do deal with him. Fortunately, he hasn't been aggressive.
My dad always loved kids. Whenever there were kids around, he was there giving them attention, playing with them. Now he has a grandson, my son, 3 years old, and is the kid he gives less attention and plays less with, due to his illness. Life really is ironic.
I'm sorry to hear this.
My mother is in her 80's and after a stroke (multiple) has slowly lost much of her memory, vocabulary, and interest in life. She used to be quite vocal and alive but now talking on the phone is almost like pulling teeth -- even then not knowing whether she's processing half of what's being said. I try to get her interested in her old hobbies, but nothing. Whether through frustration, depression, or a failing mind (or all the above), she is distancing herself from me, my dad, and life. It's heartbreaking.
I know this isn't exactly the same as your situation... but your experiences with your dad really resonated. 61 sounds so young for these problems.
Take good care and spend as much time as you can while you can with your Dad. Wish there was more I could say....