Letting 16-year-olds vote sounds like a horrible idea.
Only if you're a fundie.
There's plenty of non-fundie reasons to oppose voting at 16. In this case, it was a pretty transparent ploy by the current ruling party who wants to stay in power (as ruling parties do) and is steadily losing voters. They have a relatively powerful youth organization*, and voters between 16 and 18 have optional voting (it's mandatory in general), so they expected most teenagers wouldn't bother to show up because lazy, and those that did would be the members of their organization. It seems the overall effect on the elections was insignificant, though.
And that was your weekly lesson on Argentinian politics. Next time, we'll cover why the current format for the primaries is a total sham.
*the only major party that does, as most politically active teenagers that aren't with them tend to side with the more leftist parties that have nearly nil chance of winning anything.