I'd say 18 months.
My first I sent at 18 months because I had to go back to work but my second I've decided to stay home a little longer. He will start when he is 2. I am a little worried it might be harder for him starting older but I try to keep them busy and go to lots of playgroups so he is getting well socialized. My 18 month old did fantastic. But it probably depends on the child. They're all so different.
It's honestly a personal choice. My oldest I waited until 2 years. My second I waited until 18 months. And I will be sending my 3rd baby at 18 months as well. I feel like there's no right time but I would for sure not do it under 12 months.. that's just my choice