It has to do more with how the original Japanese translation describes the relation Tao has with both Wendy and Diane. All three of them are born in Bustoke and are considered siblings. By this logic, it would mean that Wendy must've been a baby sister to both Tao and Diane and since all 3 belong in the race of elves, they have a longer lifespan so technically it makes sense in a fantasy sort of way. Who knows how old Wendy could be?
The link I gave you, there is another link provided by Omega Entity that will take you to another forum which discusses the translation issues of Shining Force. Hopefully, it should make sense to you although you must take it easy and just understand it's all fantasy.
Here's the link to the topic 'SF Character Translations':http://forums.shiningforcecentral.com/v ... =3&t=16147
Also, the link I provided you says that Tao is in her 40+. XD