What are the factors that determine the speed flutter appears on wings?
It's an aeroelastical phenomenon driven by pitching moment, torsion stiffness, and inertia of the wing. It's a classical spring-mass-damper problem: increased aerodynamic pitching moment causes torsion on the wing structure, which causes more pitching moment until the local airfoil AoA is such that the air flow separates. Torsion is counteracted by the wing torsional stiffness, thin wings have less torsional stiffness. 

Parameters that determine flutter speed:


Wing profile: aerodynamic $C_m$ and wing thickness
Wing aspect ratio
Number of jet engines mounted forward of the aeroelastic axis
Wing sweep