I think that too much focus is placed on age as a compatibility issue in a relationship when the gap is from 5-15 years. I was with someone younger at the age of 18 and someone older at the age of 20. Both relationships were long-term, but neither of them worked out (though I'm still friends with both). In the end, it was never the age difference that was the problem; it was the fact that we weren't in the same place mentally and eventually didn't want the same things or put the same amount of effort into the relationship. I've been with my current partner for a few years now, and have never been happier because we're both secure enough in ourselves to talk to each other if there is a problem and discuss what it is we need.
What really needs to be judged here isn't the age gap in itself, but whether the age difference will make it difficult for the partners involved to communicate honestly about what they want and need from each other. The question is whether or not they are in the same space mentally and whether they want similar things. To give an example, an 18 year old who wants to go out and party constantly and a 26 year old who is focused on the future and their career may not be on the same page. This is both a case of maturity and a case of both people being at different stages in their lives. It's important to establish what both partners want if the relationship is going to be anything more than casual. Relationships don't just happen; they take work, and sometimes they may not work out. So before the risk of being in a relationship is taken, both people should give themselves a fair chance to understand one another. If on a casual level neither partner is able to communicate what they want from an eventual relationship, then it probably isn't going to work on a long-term basis.