I hope you're picking Irvine for more than just schools.
While they are excellent... they are not perfect, and as USC says, other school districts are just as (maybe even better) like TUSD that serves the Tustin Ranch area.
As for your not too old, not too young, neighborhood feel... that's subjective. Even within the same tract, some streets might be more "social" than others. Like USC, since you are not from this area, it may be best to come out, visit certain areas, and spend some time figuring out what works best for you. An SFR rental may be a good way to "try before you buy" but I understand the pain of moving, changing schools etc.
The older neighborhoods tend to be more "social", but then the demographic might be older. The brand new ones also have good "neighborly" communities... but then they might be too young. There are some members here who feel that Irvine typically is anti-social and neighbors don't talk to each other... but I find that more reflects individuals than the entire neighborhood... people respect your privacy... but if you want to be more "neighborly", they will be.
Of the new neighborhoods... I recommend Woodbury (because of the proximity of amenities) and Quail Hill. Of the older neighborhoods... I like Turtle Rock, Woodbridge and Westpark II. I also like some parts of Northwood and recently have begun to get an appreciation for University Park (although I don't like their home architecture). For not so old, not so new areas, you should loo at West Irvine and Oak Creek. Tustin Ranch is also a nice area, although not Irvine. It might also behoove you to see what area of Irvine is closer to where your jobs are at because as the city gets more crowded... traffic turns a cross city trip into a lengthy one.
Good luck and welcome to Talk Irvine.