A downhill where a road bike can reach that speed is pretty normal. I reach those nearly every day on my to work commute and I live in Iowa - there is a reason we are known for being flat so if I can find a hill to reach those speed surely anyone else can too. Of course I am riding a road bike, kids and mountain bikes may have limitations (tires?) that slow them down.

Reaching those speeds on level ground doesn't seem possible for a normal human, but level ground is rare.

I can hit 45 mph going down hill on my 1980 road bike... but that's just balance, carrying enough potential energy, being too stupid to slow down to a reasonable speed, and having hills that I have to walk the bike up most of the time. There's some skill and physical conditioning there, but not a whole lot; at that speed, there's a lot of instant feedback on form, which helps encourage one to get low and tight.

I think if we're talking about how fast you can get your bike to go, flat land, still air is implied. I don't have a lot of those conditions to try, but I'm happy to cruise around 15, and maybe push it to 20 if I don't need to save my energy for a nearby hill.

I'm not sure what you might mean by a "normal human".

I used to be a back-of-the-front-of-the-pack triathlete, with a previous history as an ultramarathon and touring cyclist. In my best shape (probably aged around 46), I was training on a flat loop course with some younger very strong but not professional cyclists where we would generally pull the group at 28mph for about between 20-60 seconds at a time.

I appreciate that there's a distance between that sort of thing and an "average person", but it's not a whole lot larger than the distance between the people who were in the group and, say, professional tour cyclists.