By MICHAEL SCHNEIDER
Source & More: variety.comThe networks still preach adults 18-49, but the Big Three are all expected to post median ages above 50 this fall -- with Fox not too far behind.According to a recent study by former Magna Global EVP Steve Sternberg, the broadcast networks as a whole have once again grown older than ever. The five broadcast nets' average live median age this year -- in other words, not counting DVR usage -- was 51.