Let's see, when Bitcoin was published in 2009 I reckon mostly people in university or early career age would be interested in something like that (with some exceptions of course), so let's say minimum of 18 years old and maybe maximum of 24 at that time.

That range today would be 32 - 38.

Over the years of course this would change, one year per year for the OG, but also there is a constant influx of new people. The key would be to know the age of the new people. I reckon it's similar to what I said in the beginning, so 18-24 would be prime age for joining Bitcoin.

So the average would be increasing a bit every year because people that stay get older and new people join at roughly the same age in general.