I don't think the difference between 20 and 26 is such a big deal, especially since at 20 a guy is quite capable of acting mature and living up to the responsabilities of having a relationship. Be careful though, because it can go wrong as well.
From my own experience: my first relationship was with a guy 6 years older than me, I was 17 at the time and still in high school, he was 23 and just graduated. Everything was wonderful as we were madly in love, but the age difference, and above all the fact that we were both in complete different stages of our life, finally got the upper hand and he broke up.
I was torn at the time, but in retrospect I understand why he just couldn't commit to someone who still had to discovere everything in life, while he was already more or less willing to settle down.