Just like in some previous posts that I have made in the past, I am going to make a shameless (but adamant and enthusiastic) plug for Match.com(.ar).
I was 26 years old when I met my then 29 year old future Argentine husband on Match almost 3 years ago. As of September 26th, we will be celebrating our one year anniversary.
Match does not really cater to a specific age group (I saw many people of both genders in their 20s who were looking for relationships). I had actually signed up for the 6 month membership and met my husband about 3 weeks after my sign up date (first and only person that I met from there, but when you know it's right...!). Despite this, I am sincere when I say that it was by far the best money that I have ever spent. This is also due to the fact that I knew I was in Argentina for the long haul, and I took it seriously.
While I didn't try many dating sites, I can tell you that if there is one website that you need to AVOID like the plague, it is Craigslist. You will really only find other temporary expats there, and they are normally not the type of people who are looking for anything even remotely serious.
So, in conclusion, you should really give Match a try. If you have any specific questions or doubts, just send me a PM.
Suerte!