Susan Elizabeth Norman, of Grantley Place, Harrogate, was last seen by a friend on the afternoon of Thursday 4 August 2011.
It is believed that she may have traveled to the Bradford area of West Yorkshire and officers are growing concerned for her safety due to her age and the length of time she has been missing.
Susan is described as white, around 5ft tall with a thin build, dark purple shoulder length hair and blue eyes.
When she was last seen she was wearing black jeans, a pink top and an aqua blue shell suit jacket. However, it is believed she may have taken a change of clothes with her. Susan also wears a silver necklace.
Police are urging the public to contact them if they see Susan or have any other information that can help the police to locate her.
PC Rebecca Wood, of Harrogate police, said: “Susan has not been seen for sometime now and we are growing increasingly concerned for her welfare.
“It is important that we find her so we and her parents can be sure that she is ok.
“I am appealing for the residents of Harrogate and people living in West Yorkshire to keep an eye out for Susan and contact the police if they know where she is.
“I would also urge Susan to contact us if she sees this appeal as we need to know where she is and that she has not come to any harm.”
Anyone who can help the police with their enquiries is urged to contact Harrogate police, on 0845 60 60 24 7, quoting reference number 12110130503.