There are a number of factors that determine the outcome of a fight, with age being one of them. As we age past our physical prime, our reflexes slow and our bone density decreases. In pro fighters, older fighters can't train as often or as intensely as their younger counterparts, and years of taking shots to the head mean that their ability to take a punch is diminished.
In the situation you're describing, assuming other factors are nearly equal, a guy in his early 20's will still be maturing, while one in his 30's is in or just past his physical prime, and will have more "grown man" strength. Though not directly related, an older guy may be calmer and have more experience.
Other factors generally matter more than age, like size, training, experience, and fitness, but age does play a factor, either directly or indirectly.