Now if both were born in 1981:
2000 when both turned 19: a peak Kuerten takes the 2005 Nadal at the French most likely.
2001 when both turned 20: since I think Kuerten would have been closely matched with even the 05-06 Nadal on clay and I gave Kuerten 2000 FO I will give Nadal the 2001 FO. I dont think the 06 version of Nadal would win Wimbledon with Agassi, Ivanisevic, and Rafter all in the late rounds.
2002 when both turned 21: The field this year was a joke, joke, JOKE. The worst year of mens tennis ever. Johansson winning the Australian Open, a past his prime Costa the French, the worst Wimbledon in mens tennis history, Schalken one of the top men in the World this year, ugh. Nadal wins the Australian Open (yes the same Nadal that got spanked by a playing out of his mind Gonzalez). Nadal of course romps to the French Open. And Nadal easily wins Wimbledon which was won by Hewitt over Nalbandian. Sadly injury probably costs Nadal the U.S Open title and a Calendar Slam which was contested by an aging Sampras vs aging Agassi.
2003 the year both turned 22: Nadal doesnt run into a tournament of a lifetime Tsonga or a top form Djokovic but instead gets a 33 year old Agassi, Schuettler, injured Roddick, or 30 something Ferreira. 2008 Nadal wins the Australian Open again. Nadal wins the French again though Ferrero gives a decent final. Same as 2008, Federer in a 4 or 5 set thriller in the Wimbledon final. I say 22 year old Nadal would not be able to overcome Roddick or Nalbandian at the U.S Open this year.
2004 year both turned 23: Nadal no longer burdened with the marathon the previous day vs Verdasco takes down even a younger Federer in the Australian Open final this year. The field at the 2004 French is so pathetic Nadal wins with his injury too before withdrawing from Wimbledon. Federer easily wins Wimbledons and U.S Open just as was.
2005 year both turned 24: Safin wins the Australian just as was, Nadal wins the French just as was when he was turning 19, Nadal ekes past Federer in the Wimbledon final (like my original breakdown when I split the 08 and 2010 finals between Nadal and younger Federer).
So in this scenario they would have:
Nadal- 5 French Opens, 2 Wimbledons, 1 Australian Open
Federer- 2 Wimbledons, 1 U.S Open
Federer actually fares better comparatively IMO if both were born in 1986. He avoids Nalbandian at the 03 U.S Open, Safin at the 05 Australian Open, then gets Nadal coming off a 5 setter in 2009 (now with 22 year old legs) rather than not as in 2004.