That strikes me as an oversimplification. According to the NYT article [1], Xerox filed an "unfair competition" lawsuit, which was correctly dismissed (as Apple never tried to mislead consumers about whether they were buying an Apple computer or a Xerox computer, among other things).
The only reference to age I found is, according to a lawyer quoted in that article, "Xerox had waited too long to file a copyright infringement case and had to resort to a weaker charge of unfair competition". It looks like the statue of limitations on copyright infringement was (at the time) 3 years [2].
It's not clear to me if this is true or not. According to the case itself, "Xerox argues that an infringement action would not afford the relief it desires". (Is that lawyer-speak for "oops we forgot to file the right charges in time"?) Either way, I can find no record of a copyright infringement case brought by Xerox against Apple.
[1]: http://www.nytimes.com/1990/03/24/business/most-of-xerox-s-s...
[2]: http://www.copyright.gov/title17/92chap5.html#507