mhm I think 2021 sounds a pretty far distant prediction tbh, considering that it was announced in 2017, that Relic has mentioned 2019 will be an important year for them, and that they recently started looking for brand lead and marketing manager.
Also, I keep in mind the interview of Bruce Shelley that @IamDalv brought us a year ago .
Where he says at some point on why he is not working on aoe4:
"I really enjoyed the time with Age of Empires, but you know - with age, priorities shift. (…) I could not move all the way to Vancouver two years or so to develop an Age 4. (…) I enjoy closing time at seven, vacationing, exploring African countries (…) things that are incompatible with a AAA title
So if I understand correctly, Bruce Shelley thinks that 2 years or so would be a reasonable developing period for a new AAA RTS title, and specifically a new Age title. I think there are not many people out there more eligible to give a prediction on that matter.
So, I would say early in 2020 or even maybe later in 2019(?) sound more possible.