Reaction time, strength (especially those who have suffered many injuries), and quite literally physically limitations. It just takes longer for your body to distribute oxygen to the necessary muscles and tissues aka VO2 max. It’s also a mental battle. Everyone around you is faster, stronger and younger and sometimes not being able to compete at the same level of intensity can be a challenge because it takes longer to recover while having to work twice as hard to maintain muscle mass. Athletes peak around 27-32 and VO2 max decreases about 1-5% every year, although not linear.
Slow twitch muscle fibres generally stay the same, but fast twitch muscle fibres shrink in size with age, which can affect explosiveness and obviously speed.
Wear and tear. Injuries, pain, fatigue, age all cause reduced flexibility in the joints and tendons which affect how quickly signals travel from brain to the nerve fibres in muscles and tissues, which directly correlates to a decrease in reaction time and hindering of reflexes. In hockey, this can be seen with the inability to protect pucks/getting knocked off the puck easily, not being able to get a quick shot off while in stride, decrease in puck control which affects split second decision making (arms and wrists aren’t able to use full extent of it’s range), and most importantly, skating.