Lol, Munsu I didn't think of Isshin specifically because of his age. He appears to be atleast from Kisuke's generation if not older.

I didn't think of Toushirou either since I believe it was mentioned at some point that he joined the academy after that girl he is so fond of had joined, whats her name Momo I think. In fact it was five years after she had joined and Matsumoto was the one who made him join so that would be much much later probably even after the period when Renji and the others were in the academy since Matsumoto herself had already graduated. So he is really young he just rose through the ranks really quickly.