The reason for the large amount of "kiddy" feeling games is that many kids have made games (though most of them have been unpublished by our mods because those storygames generally are abhorrent.) An 8/8 for maturity rating would be most of anything that is--to put it in public-friendly terms--of an adults only nature.
If you want storygames that are of a high quality and do not feel kiddy, the works by EndMaster, Steve24833, or almost any of the stories in in "top storygames" category should suffice.