I would share Paulette's comments as well. Outside the city, I would throw in Conshohocken, West Conshohocken, Ardmore, and Bryn Mawr as places "trending up" with millennial renters. These areas are all connected downtown via rail and have close proximity to a number of schools, but have a slightly more mature population compared to say Manayunk. If you are looking for areas where millenials are starting families, I would look add in Phoenixville, King of Prussia, Media, Plymouth Meeting, Collegeville, and even as far out as Malvern (in addition to the aforementioned areas in the city).
I would consider Overbrook a hybrid area. It is a tiny geographical area that touches the city limits. I would lean towards an urban feel given it's proximity to downtown Philly.
I am 30, and purchased a 2/1 row home in Conshohocken with the intent of renting it out in 5-10 years once starting a family.
The county planning documents are a great read in terms of finding areas with planned improvements. For example, there is a detailed Montgomery County plan to put in a additional rail line to King of Prussia by 2023. Philly Business Journal (regional), and Morethanthecurve.com (conshohockens/Plymouth/Whitemarsh) are other publications worth reading for insights.