As most people said, around the 30-40% mark is brilliant for longish videos. For shortish videos, you should aim around the 50% mark or above.
I am not gonna say 21% is bad, since I averaged around 12% (Yet I don't use or care about my channel) , but you might want to consider lowering the length of your videos.
Long videos sometimes puts people off clicking on the video. 15 minutes is mainly the optimal length, there is enough gameplay (or whatever your channel is about) in the video but it can also be watched within small breaks.
Personally, that percentage is good, but the next step is to analyse the reasons why people may leave.
Given that your videos are approximately 3-4 minutes long, somewhere between 1:30 and 2:00 is the stopping point.
Do you drag on? Are you not enthusiastic enough? - Look for the differences between the start and then.