I have thought about this for a while now. Even though I have enjoyed anal pleasure (using various toys including dildos) since a teenager, I never thought about a "real" cock until my late 30s. Also, for me at least, it isn't an attraction thing nor a sensual thing. I am not interested in hugging, or kissing, or raising cats with a dude. For me it is more about "taking cock" and the mental gratification of making a man cum with my mouth and/or ass (on top of my physical pleasure). This tells me that my bi-curiosities are submissive in nature. I have not taken "real" cock YET (but I have taken lots of strap-on peggings), so I know it is just a matter of time before I do take real.
I also believe in the Kinsey scale, and that 99% of the population (men and women) are in the 1-5 range.
So to conclude, I think many reasons cause men to think about bi sex as they age (many if not all already listed/posted):
1. Care less about labels
2. Look for other ways to have sex if/when their wives/girlfriends are less interested
3. Testosterone levels drop
4. Kink!
5. Dominant and/or submissive thoughts/feelings
6. Less taboo
7. More open minded/wiser
8. Less worried about masculinity
9. More women (at least it seems to me) are turned on by m/m play