Videl was born in Age 756 and Pan, Videl's daughter, was born in Age 779. This means that Videl was 23 when she was pregnant. Pan was born at the very end of Dragonball Z. And Dragonball GT took place OR 5 year after (Daizenshū), OR 10 years after (FUNimation). If we put all that together, then we get that Videl was between 28 (Daizenshū) and 33 (FUNimation) years old in Dragonball GT.