Definitely before school age. That way his “street” vocabulary will be so far ahead of everyone else he’ll never get mithered
I was 6 when I started going with my late Father and Grandad, around the 80/81 and 81/82 seasons, I was spoilt from the start. My own son, now 18, went to his first home game aged 4 in Sep 05, it was the 2-0 home defeat to Doncaster when we had Austin and Kay sent off by Uriah Rennie. Our only home defeat that season which ended up with promotion under Andy Ritchie, he fell in love with the reds immediately. He'd always kicked a ball about from toddler age and spent 90 minutes concentrating on the game and offering his own views to Row BB fellow supporters too, never had to entertain him through boredom. Seriously, there's a lot of positive interaction for kids with those around them and watching football, it was one of the best things I ever did taking him to that Donny match. He's had a season ticket ever since 06/07 has travelled with "the big misters" to away games and now goes with his own mates. He goes to University in Birmingham (fingers crossed) in September, but at £95, got a season ticket again as he'll cover the cost with the games he can get to during University holidays.
Took my lad to his first match at 6. He was probably about 9 before he could sit and enjoy a full game. Before then he needed lots of sweets and his gameboy to keep him entertained... It’s worth starting them young though. I get as much pleasure watching him cheering on The Reds as I do watching the match. Plus at 11 he’s already a long time season ticket holder, seen us demoted and promoted twice, been to Wembley, and been a mascot. His cousin (18 months older) is a Man U fan and has manged to get to just a handful of games at Old Trafford over the same period - and at significantly greater cost.
My nephew started taking his daughter the watch Sheffield United from about age 4. She has stuck with it throughout and now at 19 she is Manchester City and England Goalkeeper.
I started taking the little one at the age of 3Yrs, I will never forget his face when he saw the pitch from the Eaststand upper. I would take a bag of food and toys as board ness sets in. The reason I start taking him so early was due to him starting FooTballl at the age of2yrs
Mine went from 5. As soon as the free season ticket kicked in. They're not free any more but at ten quid still a bargain. He can then go if/when he wants but you've lost very little if he doesn't take to it. Snacks and something to entertain (a child friendly phone app with a well charged battery can be a lifesaver) are essentials at that age. Nevertheless, if you're asking what age I recommend taking them, unless you want them to follow in your footsteps of having enjoyment (or otherwise) of 40 weekends a year being determined by events you have no control over, may I suggest about 90?