Can a 50-year-old man be as physically as strong and fast as a 30-year-old man?
By the way you worded your question, you make it an easy "yes". :)
Obviously there are a huge number of 50-year-olds who are in better shape than 30-year-olds who do nothing to stay in shape, who have health problems, or even ones who work out, but ineptly.
Maybe you really mean "can a 50-year-old man be as strong and fast as a 30-year-old man who has the same body type, does the same workouts, and has the same level of health?"
The answer to this would be probably "no". A younger person will tend to find it easier to stay in top shape than an older person. They will tend to recover faster, and will tend to find it easier to be strong and fast.
That's the bad news. Now the good news: a 50-year-old who has a better workout plan, a better diet, better sleep, and more persistence than a 30-year-old will probably smoke the younger guy, it happens all the time.