The tricky part is that this character, and others like her, exist in a grey area of cultural acceptance and morality.  In Japan age of consent is 13 yes?  So a 13 year old girl is probably largely looked at the same way an 18 year old girl is here in America.  That type of localization and/or censorship I feel is just fine.  Make the age of the character you want to sexualize legal for the area in which you are marketing, or remove the sexualization.

However, with the fact that a 13 year old and an 18 year old more often than not look VASTLY different from one another, that raises another issue all together.

It's an invisible change that some have opted to do. Honestly, if they never told the player on the age of the non-existent person was, then I don't the conversation would happen at all. At least, not to this extent.

One only has to look at Lin to see she is very clearly in her early teens. Bumping her official age to 18 wouldn't have solved the problem.

Besides which, as has been pointed out, raising her age would itself be an alteration of the original game for the purposes of meeting different cultural standards. You achieve the same aim by cutting the costume entirely, with the added bonus that you're removing something that is seen as ethically objectionable by the standards of most Western nations.

Removing something isn't an intrinsically bad thing. Tumours, for example.

Obviously it is something appealing which sells the game to people who sexualize young teens. What is the alternative reason for it being in the game? Nobody has to "say" it, it is implicitly true by logic. 

Also, from what I just googled, if you kill the children in MGSV you fail the mission. So obviously that doesn't fit the criteria, "killing children was seen in a positive light."