It's just aging, things slow down. They don't slow down as much as some people like to think, though - the people who say, "oh but I can't work out and I have to have a big belly because I'm old, man."
For athletics competitions there exist age-adjusted tables which give factors to adjust for age. This is just like the various ways we have of comparing lifters in different weight classes; isn't a 69.5kg guy benching 102.5kg more impressive than a 99kg guy benching 102.5kg?
If a 60 year old sprints 100m against a 20 year, and the older guy runs it in 13.05 seconds and the younger guy in 11.00 seconds, who won? Shouldn't we adjust for age as we adjust for weight? Well, the factor for a 60 year old male doing a 100m sprint is 0.84, giving us 13.05 seconds x 0.84 = 10.96 seconds. We'd consider that the older guy had actually won the race.
These factors are always changing, because they're based on real-world performances. If the best 60 year old's 100m sprint is 12 seconds, and the best sprint at any age is 9.58 seconds, then the factor will be 9.58/12 = 0.80. So the factors don't necessarily represent the limits of what's humanly possible, but the real world performances - in all age groups, these are improving, as older people start saying, "well, maybe I don't have to just sit around feeling old and crippled but can actually do stuff."
But for what it's worth...
In speed events like the 100m sprint, the decline is about 5% through each decade after 30, and 10% from the 80s on.
In endurance events like the 5km run, it's about 6% for each decade after 30, 7% further in the 60s, and 14% and accelerating into the 70s.
In strength events like shotput, the decline doesn't set in until 35, is 8% by 40, total 15% by 50, about 33% by 60, and so on.
The declines are relative, of course. If at 60 you have 67% the strength you had at 20, well it could be 67% of a lot or 67% of not much at all. And maybe if you weren't an athletic 20, 30, 40 or 50 year old, at 60 you could become stronger than you ever were in your life. You might only be 67% as strong as you could have been had you trained at 20 - but you can still be stronger than many current 20 year olds.
Some slowing down is inevitable. But it's not as much as some people like to make out.