User blog comment:Mobo85/Questions for Aliki Grafft/@comment-4343598-20110909223747/@comment-3212462-20110909225352

These are questions to Aliki Grafft, a writer for Phineas and Ferb.

As for the line in the song you're referring too... It simply states that Ferb joined Candace's family after Phineas did since Ferb is her stepbrother, while Phineas is her blood brother. It doesn't necessarily mean that he is younger then Phineas.