Is it normal that I am a 52 y/o man, and I suck my thumb. I used to hide it for fear of embarrassment, but this last year I either have become more comfortable, or simply don't care what people think about it. I suck while I drive, occasionally in a store when no one is watching, and often when I sleep.

I used to LOVE it when the class misbehaved and the teacher would turn off the lights and make everyone put their heads down on their desks because I could secretly suck my thumb in school!!
I don't remember how or why I quit.
I don't really see anything wrong with it if you like it. If you wanted to quit, I know they make nasty tasting nail polish for nail-biters, or you could use some sort of hot pepper stuff, that would probably discourage doing it.