When Ross and Rachel were having their baby, Janice was put in the same room and revealed that she remarried and was having a son. ("The One Where Rachel Has A Baby, Part 1")
Janice later goes into labor and has Aaron Goralnik. Janice introduces Aaron to Rachel and Ross with Ross later asking Rachel "Did you see the kid on that nose?"
Aaron and Emma Geller-Green, Ross and Rachel's daughter were born the same day, meaning that Aaron's birthday is May 16th, 2002, just like Emma. It is also possible they would end up going to school together and be in the same grade.