She was isekai'd so she's mentally older even if she's naive.
Also, it seems like the plot is set back in the times when 30 year olds and such would marry 12 and 13 year old wives right out of debut. In all honesty it's not that bad if you choose to read the story in the time it was meant to be set. Again, also the fact that she is isekai'd and is actually older.
The drawing style also makes it much worse by drawing them to look so dramatically dofferent in age, BUT again, she was raised in an orphanage so I wouldn't be surprised if she was malnouished.