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globecanvas +1
Also, many times masters men and women prefer to race together even if the open younger categories are easier. And there is always a fresh supply of younger very strong but less skilled racers starting in Cat 5 that can give the appearance that the younger racers are so much faster. Bicycle racing has a very high skill factor such that pure strength is alway good to have but not enough to be successful at racing.
When I started racing 10 years ago, I enjoyed riding with the young cat 5 men. I would select either the Cat 5 or masters Cat 4/5 depending on the time the category raced. Sometimes the masters raced later in the day and the cat 5 men in the morning.
The key to successful bike racing is to focus on skills to avoid injury. One can never have too much skill going into a race. Off the back is perfectly acceptable whereas on the pavement is not. YMMV.