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"His" is entirely grammatically correct, even if he was speaking about teens in a general sense, both male and female.

While not grammatically incorrect[0], it is stylistically incorrect, unless the writing style you're going for is "jerk".

[0] Major sources (MLA, CMos) have made nongendered pronouns suggested

it may be grammatically correct, but it is still exclusionary.

i don't know why people fight so hard to just simply not include others

Not at all, it includes women too.