Not nearly as likely as when you were younger, but still fully capable. As a man gets older his sperm count decreases, and the amount of mature sperm also decreases. While this makes it more difficult for a man who is older to impregnant a woman, it is still entirely possible. The amount of sperm in ejacualte fluid is only about 75% of what it was in the early 20s-late thirties, while the amount of mature drops to about 50% for men over 40. 

Here's a paper on the subject:  http://www.infertile.com/inthenew/sci/maleage.htm