If you have scar tissue on a knee can a mri scan tell how old it is?

Not usually: MRI usually can't tell the age of a scar.

Not really: Once an injury has had time to heal, there may be changes indicating a chronic or remote injury. Then you cannot tell how old the injury is. For example, if the injury heals in 6 months, then you cannot tell if the old injury is 6 month old or 6 years old, based on the images. The clinical history of the patient is more important in determining how "old" an old injury is.