["I'm sad. My youngest son starts kindergarten tomorrow!", 'I am sure it is a bittersweet moment.  I can relate myself.  It takes some getting used to', "I mean i'm happy for the up coming peace and quiet but it's still sad all my kids aren't babies anymore."]
You will still have plenty of time before your son becomes an unruly teenager.  Cherish it.
My youngest son is going into kindergarten and I'm a little down.