If her eyes are open and her ears are fully upright and her fur is developed she's likely old enough to be away from her mother. That usually happens between 15 to 20 days of life. It might be a bit young to be weaned but if she's developed enough then she will manage. Make sure she has food she can eat and a water bottle to drink from. You really do need to consider getting her a cagemate though - another female about her age. Rats rarely do well when kept alone. They are just far too social to thrive as a lone rat in a cage even if you give them tons of human attention, they still need another rat to socialize with, share grooming, snuggle with, etc. Please consider a friend for her.