>>28311790The official timeline posted by one of the developers is pic related
I'd say BW takes place at most 5-6 years after DPPt/HGSS. Red is canonically 11 in RGBY, implying Blue is too. Which means Red and Blue are 13 in HGSS, and 5 years later, Blue would be 18. BW2 occurs 2 years after BW, which makes Red/Blue 20. We know that Blue went to school abroad in Kalos, so him being 20 makes sense seeing how he is still in college.