What’s so special about 40?

Why Noah made the Ark in 40 days? Why the rain/storm lasted for 40days? Why the Israelites walked through the desert for 40 years? Why the tower of barbel was built in 40 days? Why Jesus Christ fasted for 40days, and He ascended to heaven after 40 days? Why Easter is celebrated after 40 days? And they say Life begins at 40. 
Even Pass mark starts at 40. And they also say "A fool @ 40 is a fool forever". 
Again, what’s so special about 40?