The Last House on the Left

Mari Collingwood plans to attend a concert with her friend, Phyllis Stone for her 17th birthday. Her parents, Estelle and John, expresses the concern about the band and her friendship with Phyllis, but let her go and give her a peace symbol necklace. Phyllis and Mari then heads to the city for the concert. On the way, they hear a news report on the car radio of a recent prison escape involving criminals Krug Stillo: a sadistic rapist and serial killer. His son Junior, Sadie: a promiscuous psychopath[5] and sadist; and Fred "Weasel" Podowski: a child molester, peeping tom and murderer. Before the concert, Mari and Phyllis encounters Junior when trying to buy marijuana. He leads them to an apartment where they are trapped by the criminals. Phyllis tries to escape and tries to reason with them, but she fails and is gang-raped by Krug, Weasel and Sadie. Meanwhile, Mari's unsuspecting parents prepare a surprise party for her.
The next morning, Mari and Phyllis are gagged and put in the trunk of their car and transported to the woods by the criminals. Mari recognizes that the road is near her home with a look of horror. Mari and Phyllis are forced to perform sexual acts to each other and Sadie attempts to perform oral sex on a weeping Mari. Phyllis distracts the kidnappers to give Mari an opportunity to escape but is chased by Sadie and Weasel, while Junior stays behind to guard Mari. Mari tries gaining Junior's trust by giving him her necklace and instead names him "Willow". Phyllis is then cornered by Krug with a machete and stabbed to death by Weasel. After seeing her leaning up against a tree, barely still alive and in shock, Sadie reaches into Phyllis's wounds and pulls out her insides.
Mari convinces Junior to let her go, but her escape is halted by Krug. Sadie and Weasel presents Phyllis's severed hand as Mari screams in horror. Krug carves his name into her chest then rapes her. She vomits, quietly says a prayer and walks into a nearby lake to clean herself, but Krug fatally shoots her, leaving her body floating in the lake. Krug, Sadie, and Weasel clean up and change out of their bloody clothes.
In their new attire, the gang goes to the Collingwoods' home, masquerading as travelling salesmen. Mari's parents let them stay overnight, but Junior exposes their identity and orders them to leave. Later that night Junior is in the bathroom, coughing and throwing up when Estelle enters to check on him. She gasps as she sees that he's wearing Mari's peace symbol necklace. She eavesdrops while the gang is spending the night in Mari's bedroom and finds blood-soaked clothing in their luggage. She and her husband rush into the woods where they find Mari's body on the bank of the lake. They carry Mari's body back to the house and exact revenge against the crooks.
Estelle seduces Weasel and performs fellatio on him that turns deadly when she bites off his penis and leaves him to bleed to death. Mari's father John takes his shotgun into the room where two of the criminals are asleep and shoots at them. Krug escapes into the living room and overpowers John and is then confronted by Junior, brandishing a revolver and threatening to kill him. But before he can do so, Krug manipulates Junior into committing suicide and Junior suddenly shoots himself. Krug is incapacitated by an electrocution booby-trap set earlier by John. Sadie rushes outside where she's tackled by Estelle, but she escapes and falls into the backyard swimming pool where Estelle slits her throat with a knife. The sheriff arrives just as John kills Krug with a chainsaw. The deputy then brings Estelle into the living room before removing the chainsaw from John's hands.

Question: How old is Phyllis turning?
Answer: 17