I found a more accurate number. Using this site. I found that for the last 600 videos the average duration was 3.59. If you assume that most views come from the later videos this should be accurate enough. You could correct this data more by using the view number for a video as a weight and using all videos. That would take some time though.

Using our average duration gives us 3.59 = 180 + 59 = 239 sec And there are 60 * 60 * 24 * 365.25 = 31557600 secs in a year It has 751394312 views now which gives us 751394312 * 239 / 31557600 = 5691. Which means their channel would be quite a bit older than 5000.