The basic premise of Prometheus is the quest for eternal live. It is not outlandish to think that Shaw may be granted eternal life once she reaches the Engineer homeworld. In such a scenario, she may well live into the era that Ripley is in. That and getting stuck in cryosleep for a few decades could do the trick...
I personally think Shaw is somehow related to the Yutani side of things. In the initial viral videos for Prometheus when Shaw is doing her pitch, we see the Yutani logo. Its entirely possible that Shaw becomes the leader of Weyland-Yutani in the future and in the end we see a confrontation and ultimately an understanding between her and Ripley.
Far out conjecture, I know, but it would work...