Laura Marling is like the late Alex Chilton, back when he was 16, singing "The Letter" with the Box Tops and sounding for all the world like a 45-year-old man.

Marling, the British singer-songwriter who played three shows in town Friday - one at the World Cafe Live in the afternoon, and two at Grindcore House that evening - is just 21 years old. She has already released three albums of unearthly, ancient-sounding folk songs so convincingly rendered that you have to wonder what kind of deal she's struck with "The Muse" - the title of the first song on her new album, A Creature I Don't Know.

Marling began her day with a packed WXPN-FM Free at Noon show at World Cafe Live. The diminutive blonde was as self-possessed as you'd expect from listening to her albums, which are full of the sort of forlorn story-songs, stripped of references to contemporary culture, that are more often heard from British Isles songstresses decades older, such as June Tabor and Norma Waterson.

Rather than give up personal details, Marling inhabits characters in her quietly commanding songs. But she did make one revelation during her daytime set. "Alpha Shallows," from her 2010 album, I Speak Because I Can, which has the line "the gray in this city is too much to bear," was written - guess where? Philadelphia.

At the World Cafe, Marling masterfully held the crowd rapt for 35 minutes, standard length for a lunchtime freebie. The first of her two shows at Grindcore was similarly brief, if much more intimate, held in the back room of the vegan coffeehouse in hipsterizing South Philadelphia, barely big enough to squeeze in maybe four dozen fans. They sat on the floor as the Hampshire, England, native sang her shadowy songs, her face barely illuminated by a lone table lamp.

A Creature I Don't Know rounds out Marling's sound nicely, where necessary, with banjo, cello, and violin. But at Grindcore, Marling played solo, supporting herself ably on acoustic guitar - sometimes casually strummed, sometimes vigorously attacked. With a steely voice that would slip into conversational tones, a la early Bob Dylan, she performed harrowing songs such as "Night After Night," which seems to depict approaching death through an elderly woman's eyes.

"Night after night, day after day / Would you watch my body weaken and my mind slip away?" she sang, casting a transfixing spell that made the usual romantic concerns of artists her own age seem merely child's play.