My son turned 31 and he rides 20mph+ all the time. Great workout. But I'll ride the same route and sometimes ask him "did you notice ...." and the answer is almost always
"no". One time there was a young pony and mama horse (we both love animals) and I was super excited when I caught up with him to talk about them - but he never even saw them. Too busy flying along at top speed. I suppose I could use to speed up a little bit (usually 12-13mph with bursts up to 16mph) but he could use to slow down a little to take in the environment. Might as well ride inside a gym on a spin bike if you aren't going to pay attention to what's around you.