Instagram....
I've had an arbitrary age limit of 13 in mind for most social media, but it hasn't become an issue yet (my kids are 10 and 3). Even at that point I would feel a responsibility to monitor interactions pretty regularly, I think. This is all hypothetical until the kids get older, of course. Easy to plan things out before you've actually dealt with something.
I would agree with the above recommendation to wait until a child is at least a teenager. There is so much out there that your child could run across I think they should be mature enough to handle it. My son who is 11 began asking me to get on Facebook last year and I was totally against it. I feel for one it will begin to take up too much of his time and he needs to stay more focused on school instead of social media. Instagram is not as bad as Facebook but I would still wait a few more years.
My wife and I have a similar stand point with social media. They have asked several times about FB but that is big ol' NO GO! A friend of ours has a 12 yr old son on FB and she did monitor his activity. One day she went into his inbox seeing that there was a new message. It was a man trying to contact him and included a photo! Needless to say it was a scary yet valuable teaching too for our kids. Instagram seems to be the new thing now. They do have KIK but they only have contacts we know and we are in the room when they are talking but they most often use facetime to talk to their friends. Crazy...I used to have to pick up our landline house phone to talk to a friend and if I wanted to see them I would have to go outside. Now they are daily video chatting with friends two states away and family members across the country. Its really neat if used correctly and supervised.