We haven't decided that yet (she's only 3 months old at this point). We'll discuss that when/if she decides she wants to be on social media. Hopefully by then there will have been more studies into the effects of social media use with age, and we can make an informed decision :)
I have done the same thing as OP. The kid doesn't get photos put online outside Google Photos, and those are only shared with family members who know not to leak them.
For me, old enough is "when you can demonstrate meaningful understanding of the negatives of the platform". "I want to because my friends use it" isn't enough. "I want to but I'd only use [Snapchat-style ephemeral data] so people can't make fun of me with it" is much closer, as is "I want to because I want to talk to my friends but I wouldn't connect with people who aren't my friends".
So it's not a hard number, but my guess is that it's going to be about 13, like others have said.
