Okay, headcanon time on the 5Ds timeline
YAY, SPECULATION!!
Yugi @ the very beginning of the series (High School Sophomore in "American Standards") Age 15 or 16
Ushio @ the very beginning of the series (Assuming High School Senior in "American Standards") Age 18 or 19
Yugi @ the very beginning of GX: Age 26
Jaden @ the beginning of GX: Probably 14 or 15
Jaden @ graduation/end of GX: Probably 16-17
Yusei @ the beginning of 5Ds: 18
MEANING: Zero Reverse happened 18 years before the events of the beginning of 5Ds
And assuming Ushio @ the beginning of 5Ds: 35-45
And assuming Dr. Fudo @ Yusei's infancy: 27-32
Subtracting Ushio's speculative, maximum and minimum age in 5Ds from Ushio's minimum speculative age in the 1st volume/season 0-
The beginning of 5Ds could have taken place approximately 17 to 27 years in the future, after the events of Yu-Gi-Oh! (The original manga/anime)
Either Zero Reverse could have occurred soon after Jaden's graduation, or merely a decade after Jaden's graduation.
...this could also mean that, speculatively, Dr. and Mrs. Fudo could be relatively close to Jaden's age... Then again, I know absolutely nothing about GX's timeline (I don't religiously watch GX, unfortunately) which is probably where the most errors will occur in my speculation.
Still, this is conjecture, and approximation.
Feel free to throw in your headcanons too (and PLEASE throw in some Zexal approximations, because I know NOTHING about Zexal)