According to Official Guinness Records,
The oldest first time grandmother is Marianne Wallenberg (b. 07 Feb 1913, Germany) who was 95 years old when her first grandson Joshua Fritz Wallenberg (Canada) was born at Sinai Hospital, in Toronto, Ontario, Canada, on 29 April 2008. Marianne became the first time grandmother when her son James and his wife Jacqueline gave birth to twins Joshua Fritz Wallenberg and Karina Diana Wallenberg.
For a complete list of 2008 records, please visit 2008 Guinness Records in United States.