I started taking ds to a football class at a local leisure centre when he was 4. These classes were suitable for different age groups. He now goes to an outdoor team run by dads , imo i would try and find an indoor group to start off with given football is played through the winter and it get's very cold and wet for lo's. Never mind you stood on the sidelines!
Nope - My DS1 started rugby when he was five (non contact) and DS2 who is four and two months has first rugby session this weekend. They soon tell you if they love it or hate it - obviously if hate it will stop and I always stay and offer support and snacks - two hour session!