I'm a 13 year old and I have no problem with Musical.ly. That being said, I have no problem with any other app. If you let your kid use the Internet at all, they have probably already seen some things they "shouldn't see". If you let your kid go on the Internet for 10 minutes alone, they could have seen porn, swearing, etc. Chances are, unless you monitor you kid's Internet use very, VERY, closely, they've seen some things you don't know about. *The average kid has seen porn by the age ELEVEN!* My point is that if you let our kid use the Internet freely then they should be able to use musical.ly, and if you trust that your child is responsible, then they can use this at any age because remember; if they're 11 or over, chances are that they've seen porn already! I know I have! ;)
This app lets you be creative but if you don't want kids to be able to have as much freedom, get them using iMovie! It's great!