I think I shipped my first production GraphQL server around this time 8 years ago, but by all accounts it was in use at Facebook a couple of years before it was opened up. So overall ~10 years old sounds right, though the released spec is a little different to the internal version that predates it.
It's first non-draft release was 5 years ago. [2]
It's first release under community governance was 2 years ago. [3]
[1] https://github.com/graphql/graphql-spec/releases/tag/July201...
[2] https://github.com/graphql/graphql-spec/releases/tag/June201...
[3] https://github.com/graphql/graphql-spec/releases/tag/October...