Most people dislike younger players for three repeated offences of people their age:
1) Immature/ dishonourable gameplay. That is to say, most kids that age are too geared up at the fact that they're living out CounterStrike to pay any attention to honour, respect and maturity.
2) Self-Imposed Ignorance, when someone asks a question that can be found out by a) googling it or b) checking the FAQ's but are too lazy or "in too much of a hurry" (I love that excuse, it's as if the internet's content will vanish before they ask the question).
3) Disregard for other players' and/ or their equipment. In the haste of being youthful, manners are often tossed aside.
Like you said concerning ASC's "hate for minors," there are exceptions, but they are few and far between.
Now, as for your brother playing, it usually comes down to two factors:
1) The Field Owner
2) The Teams/ Organization Involved
If the teams or organization involved are willing to take a chance and allow a minor to join in for a game, that's all well and good. However, it also takes the field owner to allow a minor on their field. Some don't for simple insurance reasons, not out of spite or hate for young players, but out of fear of repercussions for injuries to a minor sustained on their property.
So ask around. It really doesn't hurt to ask. Don't just assume a no answer will be handed down. Ask the players involved and ask the field owner. If they're willing to make arrangements for you and your brother, then great, if not then he'll have to sit and wait like the rest of the underage players.