Her biological age is different from her chronological age. Sigourney Weaver was 29/30 when she made Alien, but she looks a lot older in Aliens, when technically she shouldn't (but you can't stop the actress from ageing).
According to wikipedia, Ripley was born in 2092, making her about 30 at the time of the events in Alien. So, biologically, she would be the same age in Aliens and Alien 3. In Alien Resurrection it would have depended on the cloning process.