In Bertha’s exposition about her time spent in the convent, she mentions "cross-stitching silk roses" and specifically states, "[u]nderneath, I will write my name in fire read, Antoinette Mason, nee Conway, Mount Calvary Convent, Spanish Town, Jamaica, 1839" (Rhys 48).

This is worth noting because, like it's hypotext, Wide Sargasso Sea offers very few concrete dates. Consequently, readers much determine important dates, ages, and times relative to the other events of the novel. A specific marker of time like this quote is valuable in interpreting the chronology of the rest of the novel. 

Prior to this quote, we don't have much information about Bertha's age. However, we do know that "Louise was born in France fifteen years ago" so it can be estimated that Bertha is approximately fifteen in 1839 since she and Louise are peers and in the same convent classes (Rhys 50). From this we can further estimate that was Bertha was born in 1824, that she was about nine when Coulibri fell into disrepair and her family into poverty, and approximately fourteen when her mother married Mr. Mason.  

The narrator goes on to say, "During this time, nearly eighteen months, my stepfather often came to see me," and "[t]he last time he came was different...I was over seventeen, a grown woman" (Rhys 52, 53). This quote is evidence that she spends a little less than two years in the convent and supports the previous age estimates.

A clearer picture of Bertha’s age lends a fresh sadness to the traumas of her life. Knowing she is just nine when her family loses everything and she is under twenty when Rochester takes her dowry and locks her away, makes her situation all the more tragic in the context of her youth, innocence, and naivety.

Rhys, Jean. Wide Sargasso Sea. W.W. Norton & Company, 2016.

Silva Gonçalves, Davi. “Drowning in a Wide Sargasso Patriarchy: The Borderland’s Revision of Female Prototypes.” Todas as Letras: Revista de Língua e Literatura, vol. 19, no. 3, Sept. 2017, pp. 194–205. EBSCOhost, doi:10.5935/1980-6914/letras.v19n3p194-205.