My son is 15 months and I still quarter them. He’s a great eater. But half just seems to big. You don’t want to make them go small tho because they will just swallow them instead of teaching them to chew.
If you're worried you can always start by cutting them super thin, and / or small. It's okay to start smaller! At almost 12 months, my girl is a pretty good eater, but whenever I have been in doubt, small pieces or matchsticks has worked just fine. The reason grapes are a problem is their round shape - it's the perfect shape for blocking an airway. It also won't respond to back blows. But foods that are properly prepared will do so - in the event of an emergency, administering back blows should dislodge anything that gets stuck. Trust me, I've had to do it and it does work.
Grapes are only a danger when they are whole as they are the perfect shape to make a ‘plug’. Once quartered they are no different to any other food.
I used to flatten the grapes and blue berries until I felt my kid could handle it and my anxiety was better.