Barnwell was reported missing on Nov. 15, 1985, but investigators have long suspected she vanished before that day, according to Pennsylvania State Police.
She was last seen by her mother and is said to have gone missing from her Germantown home at 5169 Wakefield Street.
Many circumstances surrounding her disappearance are still unknown, but initial investigators suspected foul play, according to missing persons database NamUs.
Anyone with any information is asked to call PSP's Philadelphia station at 215-452-5216 or submit a tip anonymously online.
A $5,000 reward is offered for tips that lead to a break in the case.
Click here to follow Daily Voice Allentown and receive free news updates.