Now I understand all that quibbling about how many years older Charlie was than Percy and the rest! I've always struggled with what to say when describing how many years older my siblings are -- my brother was born in a year 8 years ahead of me, but he was 7 years ahead in school. My sister was born 6 calendar years ahead of me, but was 5 ahead in school.
So what? Mostly, it just makes me feel worse about poor Percy, stuck in the middle like a fifth wheel. In my family, we naturally fell into groups of two: my parents, me and my twin, and then the older two sibs. I can easily see Bill and Charlie grouping together, and of course Fred and George. Later, when the other kids came along, Ginny and Ron would group together naturally since they are the youngest AND because they are so close in age. There wasn't a big window of time for Percy and Ron to be grouped together, like there might have been if Ginny was several years younger.
So there's Percy, wanting to be included with the adults and older kids, and definitely not feeling a part of the twin's mini-group (who would? It's tough with twins in the family). No wonder he got away after leaving Hogwarts and attached himself to someone else entirely.