She maybe around 300 years old... Just a guess.
Her true age is anyone's guess, but given she returns to her home world every 100 years, she would have to be at least a few centuries. The reason she still looks so young, contrary to everyone thinking she was immortal, is because in order to be able to watch over other, more distant parts of the cosmos and still make it back to her home world within a single century, the Comet Observatory would need to travel faster than the speed of light. Thus, time passes very quickly during Rosalina's travels, but since she's in the observatory, she ages no more than a regular human being. 10:55, November 20, 2013 (UTC)