My son turned 5 at the end of December 2016. We used the local catholic church for preschool when he was 3 (turned 4 during the school year) and he is currently 5 and finishing Pre-K. He has always picked things up rather quickly, but we did not feel he needed to go to school early to challenge him. We tried to push letters, numbers, site words, etc. as much at home as we could without burning him out. My wife does a better job of it than I but then again she is a 5th grade teacher. I can't say he is at the top of his class, because he is in Pre-K and what does that really mean in Pre-K. The teachers the last couple of years have all said he is exceptional in his social behavior and academics. My point is my son seemed ready a year early (normal age for Connecticut), but we are extremely happy with all facets of his growth (academics, social behavior, and athletics) on the "NY normal" timetable.
Now my youngest son, who turned 3 this May, will be starting preschool this September at the same catholic church his brother attended. Let's just say we have to worn the teachers he's not his brother.
So, in short, I would wait a year as many have said. Good luck in your decision.