At the end of "The Husbands of River Song "it is revealed that a night on Darillium is 24 years. After this time, River goes to the library to meet the 10th Doctor.
Is River actually 24 years older in that two-part story or is there some wibbly wobbly timey whimey stuff going on?
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.
It is unclear as Doctor Who has this habit of retconning itself. Remember that River Song didn't know of the Doctor's "13th" (Peter Capaldi) incarnation as of one his incarnations counts as two (it didn't make that much sense at the time and makes less sense the more you look at it). RIght now it looks like she was at leas 24 years older but that is about all we can really say on the matter.
