In 1841, Census Enumerators were supposed to round up ages, except for children, tho' this didn't always happen to plan. This would explain why Jane's age is what you weren't expecting and why the girls have been given the same age. If you could find one of them in 1851 and later, you should have a much better chance of finding accurate ages. Do you have access to the censuses or would you like me to have a look?

Good luck! Jo