Maintaining speed up until you are 40 is pretty darn good. I believe that with proper training it can be maintained until fairly old age but it remains the hardest physical quality to maintain and it is not necessarily related to a loss of power or strength. In my opinion it might have to do with tissue elasticity and maybe a slightly slower neural drive.
IMHO you should strive to work on sprinting mechanics a bit more as well as do more mobility work to maintain optimal sprinting form and being able to stay relaxed when sprinting.
I also believe that neurologically training takes a bigger toll when you are older for some reason. So what can happen is that your training sessions might lead to more residual neural factice which can hurt sprinting performance. So you might not be slower, but in a suboptimal neural state when running.
So reducing the frequency of highly demanding sessions might be a good strategy to use.