If you look at old videos of Bush, he used to be quite a smart and sharp man, even while governor of Texas. Both highly articulate and switched on. I'd definitely put him around 120. See this for example:
http://www.youtube.com/watch?v=pw4Bhmm22xo
I'm pretty sure what we see now is a drug [censored] mind starting to age. He has a long history of drug and alcohol abuse and combined with a lack of natural intellectual curiosity and extreme job pressures, it seems obvious why he's like he is today.