No numbers, but you will need to be a bit patient for GenX to move out of the burbs. By burbs I mean outside of 128. Don't know how millennial/(what's after millennials?) would be able to live there without a car.
Lots of towns outside 128 are building more housing and growing. Billerica, Bedford, and Chelmsford all show growth between the 2010 census and the 2016 5-year ACS estimates.
The notion of GenX people moving out of the burbs is also weird. Ignoring that the youngest members of that generation are still in their 30's and have young kids, even the older ones are expected to age in place, similar to whate the baby boomers are doing today.