>>13063452FR/LG and RSE take place at roughly the same time, as evidenced by the linking of Kanto and Hoenn's Trade/PC network in Gen III.
Three years later the events of HG/SS and D/P/Pt occur, again at roughly the same time.
After HG/SS, the Rocket Grunt who stole the Machine Part from Kanto's Power Plant moves to Unova, gets married, and has a son who's a child at the time of BW. 5 years pass at the very minimum.
BW2 takes place 2 years after BW.
So by the time of the PWT:
At least 10 years have passed since FR/LG and RSE
At least 7 years have passed since HG/SS and DPPt
And 2 years have passed since BW
So Red and Blue are 21.
