Hey, Brenda. The parenthesis around Matilda's age must be in the transcription where you found the information; it doesn't appear on the actual census page. If the source or transcriber had knowledge of Matilda, perhaps they were attempting to convey that the age was either incorrect or an estimate.
That 1880 census indicates she and both of her parents were born in Virginia.
The 1900 census (Pendleton, Jefferson County, Illinois; page 6; line 82) indicates she was born May 1865 in Illinois, with the parents' birthplaces both marked with what I assume is "Un" for unknown.
The 1910 census (Pendleton, Jefferson County, Illinois; page 1A; line 36) shows her as 43 (so born about 1867) and born in Illinois, with both parents born in West Virginia.
In 1930 (Pendleton, Jefferson County, Illinois; page 4A; line 47) she's shown as 63 (so again 1867ish) and born in Illinois with both parents born in Virginia.
The 1940 census (Pendleton, Jefferson County, Illinois; sheet 2B; line 55) shows her as 74, with husband Jerry (William) and son Eugene.
The index of "Illinois Deaths and Stillbirths, 1916–1947" shows her death on 31 May 1947 at the age of 83 in St. Clair County, Illinois. Her name on that record is listed as Matilda Ann Threlkeld and her birthplace as Golcond (sic; Golconda), Pope County, Illinois. So that date doesn't jive with the 1957 death on Belcher-3675. The obituary for her husband, William J. Threlkeld, indicates that he died 17 February 1951, not 1950, and it says that Matilda preceded him in death: https://www.newspapers.com/clip/57290855/mt-vernon-register-news/. Those 1947 and 1951 dates would agree with the headstone photo you uploaded (https://www.wikitree.com/photo.php/3/3b/Threlkeld-430-1.jpg), but not the death dates on the profiles.
Good luck with the hunt!