She has to be 24 years older. From the transcript
RIVER: Funny thing is, this means you've always known how I was going to die. All the time we've been together, you knew I was coming here. The last time I saw you, the real you, the future you, I mean, you turned up on my doorstep, with a new haircut and a suit. You took me to Darillium to see the Singing Towers. What a night that was. The Towers sang, and you cried.
RIVER: You wouldn't tell me why, but I suppose you knew it was time. My time. Time to come to the library. You even gave me your screwdriver. That should have been a clue.
She clearly spent the full 24 years there (which is why The Doctor came in the first place). I think what confuses you is why she doesn't look older. What we don't know is how River ages. Remember that she once had regeneration energy and has some Time Lord DNA. From A Good Man Goes to War
VASTRA: [snip]Now, I have a question. A simple one. Is Melody human?
DOCTOR: Sorry, what? Of course she is. Completely human. What are you talking about?
DORIUM: They've been scanning her since she was born, and I think they found what they were looking for.
DOCTOR: Human DNA.
VASTRA: Look closer. Human plus. Specifically, human plus Time Lord.
In other words, it's entirely possible her unique physiology keeps her looking young.