In general you should probably allow people to opt out of alcohol ads.
Or violent films.
It's weird that the film has an age rating; the ad has an age rating; the content has a much younger age rating; but YouTube can't see that many parents don't want an 18 / 15 movie ad on content they're showing to their children.
And in general, stupid content is far, far more worrying than "inappropriate" content. I'd rather have my kids see some nudity than many of the "kids" shows out there.
Many television broadcast regulators limit the amount and type of advertising presented to children (including the very liberal FCC), and in some cases even prohibit such advertising altogether (e.g. the province of Quebec in Canada). So far, regulations have not caught up with the Youtube generation, even though many parents treat streaming media equivalently to broadcast in how it is used by their children.