When bacteria culture age does it appear gram negative or gram positive?

Typically, a negative result can be doubted, but a positive staining cannot be. This is because the vast majority of bacteria that do revert (not to say that the majority of bacteria DO revert) will go from positive to negative. This is due to a decreased average level of peptidoglycan synthesis in Agar culture, as opposed to natural function in vivo. The decreased level of peptidoglycan allows the ethanol/acetone decolorizer to more quickly flush the Crystal Violet from Gram+ cells, making them appear either weakly positive, or fully negative.