The most dominant swimmer in American History shows no signs of slowing down as he enters the 2016 Olympic Games in Rio. Though Phelps went through a bit of "off the field" trouble following his dominance in London, he has recommitted himself to the work ethic that brought him to the forefront of competitive swimming. Many think that the birth of his son Boomer helped him mature and refocus, leading him to victories in the 200-meter butterfly, the 200-meter individual medley, and the 100 meter butterfly at the U.S. Olympic Trials in July 2016. Phelps seems to be sustaining his dominance, using his new motivation of being a father to help guide his hard work.
Michael Phelps set record after record at the London Olympics in 2012. Even though he is getting older, there is no reason why this would be a factor with his ability to perform with Team USA. Another "old" athlete was a woman from Ukraine who successfully competed in gymnastics at the age of 41. Commonly, gymnasts retire much earlier than this, but the fact that she was able to keep going bodes well for Phelps to continue to succeed.
Michael Phelps will likely compete at a very high level during the summer games. Therefore, no, age will not be a factor for Phelps during the Rio Olympics. Phelps is a high-trained athlete that has one more gold medals for swimming than any other athlete in history. His age will not likely play that big of a factor during his swimming competitions.
Age won't be a factor because Michael Phelps is still one of the best swimmers in the pool. He's in an unprecedented fifth Summer Olympics, and he proved his worth during the U.S. Olympic Trials. This will surely be his last hurrah, though, as Phelps probably won't get back in the pool after his time in Rio.