If you do not like "lustro" you can say "zwierciadło" but it's old-fashioned word.
Nah lustro is awesome because I can pronounce it ;D Thanks for the extra word anyway :)
It appears it is old Latin word lustrum that denoted ceremony performed every five years.
In Polish one (if he had to) would use pięciolatka. It came from Soviet 5-year economical programs, because it normally means 5 years old girl.
I forgot the most obvious version: pięciolecie.