My DS is 9 and, like most kids his age, loves watching youtube, especially youtubers like DanTDM, Guava Juice, Jerome, etc. Mostly, they seem pretty harmless (even thought I can't understand the appeal of watching grown men playing games, but then I am not 9, so...)
I'm just aware how it's easy to stray into watching stuff that could be too adult. For this reason, I'm deliberating whether to ban youtube and to ask him to watch 'normal TV' instead. We have netflix, amazon prime, etc so it's not like there's nothing on TV!