Mike Vick, Darrell Green, Desean Jackson and Prime Time all had elite speed and were probably the fastest guys in the NFL when they were in their primes. All four show up on top ten fastest guys in the NFL all time lists. Hurts is not at that level, his combine was 4.59 which didn't even put him in the top 10 fastest guys in his draft class. Hurts is fast enough, but he is no all time elite like the other guys. If those elite guys lost a step in their 30s, they would still be faster than most people in the NFL. They probably lost a step or two in their 30s, and certainly wouldn't have recovered like they did when they were younger.
Very few if any track and field sprinters peak after they turn 30, and certainly not their late 30s. You could always find an outlier, but an argument cannot be built around an outlier. In distance running, you see a lot of the faster twitch athletes move up in distance as they get older, where you might have someone who runs the mile in their early-mid 20s move up to the 5000 in their late 20s, early 30s, and then keep moving up in distances as they age out of the shorter distances.
I think the argument that a running QB takes a step back as they age is valid. If a QB wants longevity in this league, they need to develop as a pocket passer who can read through the progressions. If they can compliment that part of their game with a scrambling ability, great!