There's no questioning his finishing and he still has decent acceleration going for him, but I did notice the other day that a 35-year-old Michael Carrick - who was on the field for over 80 minutes - out-paced Agüero who was just brought on as a substitute.
Here's a gif I made;
In Agüero's defence, he does make a slight detour to get to his desired destination, but on the other hand he also gets a head start ahead of Carrick, is 7 years younger and had the advantage of having fresh legs as opposed to veteran Carrick who was on the field for almost 1.5 hour.