Although the red-haired songstress is eight years older than 18-year-old Niall, she revealed the pair hit it off when they met recently and she would love him to become her toyboy.
She told We Love Pop magazine: ''He said he was going to marry me. I love him! I accept; you can put that in print. He can be my toy boy. I wouldn't mind having a younger boyfriend.''
A source said: ''Niall told his family he's crazy about Demi.
''He's already going to have to send at least six months a year in the States with One Direction, so he wants to base himself near her and make a proper go of it.''
Demi recently admitted she thinks Niall is ''awesome''.
When asked if she is dating the singer, Demi joked: ''I don't even know who One Direction is.''
However, she quickly added: ''I'm kidding. Niall is a friend of mine. I do know who he is. We're just friends and he's awesome. He's cool.''