Mine are 9 (nearly 10) and 13 now, and it does kind of creep up on you gradually.
It was things like seeing a Cbeebies magazine and realising I didn't know anything about most of the programmes referred to (thank God)
Filling in reviews of holiday stuff and realising we didn't fit under 'families with young children' anymore once my youngest was past about 8.
Cleaner mixing up mine and eldest's shoes 
Getting ads on social media for events and things and thinking 'Mine are way too old for that', in fact generally seeing lots of stuff your kids are too old for.