Mea Culpa. 
In addition to which, being Mrs. Hamilton gave her plenty of access to Mr. Wilkes, being his sister-in-law and all. Lots of opportunities to moon over him. Another point in Charles’ favor. Marrying a Tarleton and she’d probably see Ashley very rarely.
She was 16 in 1861. She was born the same year Atlanta was named Atlanta, which I think was in 1845. It the end of the book she’s 28.
And while Charles may have been pushed to declare his love for Scarlett by the war, I think he was already halfway there by her flirting.