Not true. Wake turbulence can be encountered at any time - at any altitude. Its simply the point of passing through another aircraft's vortices... which can trail out for at least a couple of miles. I've experienced wake turbulence at 35,000 from an opposite direction heavy passing 1,000 above.

However, the effects of wake turbulence are most dangerous during takeoff and landing due to the close proximity to the ground and lowers speeds (particularly during landing when Vapp is closer to Vstall.