I plan to change my 7 month old to a convertible seat this spring. So he will be about 9/10 months old. I'm waiting until the really cold stuff is gone since his car seat keeps him warm.

I changed him into a convertible car seat at 4-5 months (still rear facing)

We had a from birth up carseat with my youngest and he was rear facing until 3 and a half. With my oldest he was 8 months. And with my first baby I was young and dumb and put him forward facing.