An old trick I once used was to make the video as unlisted, this did help me to keep the video hidden away and age a bit. However, if you boost the video while it's unlisted and then make it public and it ranks and gets flagged it will cause you problems. When you make an unlisted video public, you will notice that the date at the top of the video is incorrect and will show a new fresh date. I tested this and waited to see what happened, after some hours it will then revert back to the original later date it was uploaded.
Moral of the story - don't boost the video while it's unlisted, first make it unlisted and then public, after that you can boost it while it's just sitting on the back pages and has age.
Avoid direct linking in your video descriptions as well unless the videos are old and SERIOUSLY boosted!