I took my Son to pre season friendlies when he was 3,4 and 5. This was too early. He was more interested in eating and going to the toilet (every 10 minutes).
Aged 6 I signed him up as a Junior Saint (We live in Portsmouth, so I needed to convert him early). He got 5 free match vouchers and started to sit still and actually watch the games- I did have trouble explaining off-side to him.
I then purchased a season ticket for him in December I take a bag full of food for him and he is now no trouble and really enjoys it.