If you're INSANELY motivated, maybe. At 40, it might be a bit of a stretch. If its for a couple of fights for lower level promotions it could be possible. Unfortunately, it won't blossom into a long career. Example, look at Fred Ettish. He got embarassed at one of the early UFC tournaments, came back almost 20 years later and won 2 MMA fights for smaller promotions at the age of 51. Keep in mind, dude was criticized by the traditional martial arts community, going through a divorce, his son died and STILL had the focus to go and win 2 MMA fights for small promotions.
If you're doing it to push yourself, go for it. If you think that you can build a LONG career as a professional its highly unlikely.