River’s life is inextricably entwined with the Doctor’s long before he meets her for the first time in the Library, but they’re out of step with each other; that can happen when you’re a time traveller. She was younger than she is now when she first met him, but he was older, or will be. A later regeneration perhaps.
Really he shouldn’t be surprised that she needs a diary to keep track of their meetings, so she knows what it’s safe to tell him and what has to be kept secret. As she’s told him more than once, “Spoilers, sweetie.”