I'm honestly not sure how old the "babies" PetCo sells are- I thought they used to be really, really young but not enough people were able to keep them alive, so they started selling them a little older. It's hard to get a good measurement on this girl because she's skittish (should have measured when she was still in the cup, d'oh!) but it looks like she's about an inch from nose to the end of her tail. Growth charts I can find suggest that makes her about 6 weeks to 2 months- younger than I expected, to be honest!
I agree that she'll probably have a light body when she's older tho, because from what I understand darker fish usually have visible stripes when they're young. Another baby there that I assume was a sibling had a light body and red starting to show on their fins, so who knows what may happen!