The Guinness Book of Records 2002 Edition reports that the largest age gap between two siblings is 41 years. Elizabeth Ann Buttle gave birth to her daughter in 1956 when she was age 19 and then later gave birth to her son in 1997 when she was 60.
The average human female reaches peak fertility around age 25. However, most women are capable of naturally conceiving until age 41. The oldest woman to ever give birth was Rajo Devi, who had her daughter at age 70. She conceived through the use of a donor egg artificially inseminated with her husband's sperm.