I think when your older (i.e teens and twenties and so on) it depends on your personality too....not the closeness in age thing.

There is 10mths b/w me and my sister. So we go 1975,1976,1978 and 1981.....im probably personality wise closest to sister #3 (1981)....there is a 6 yr age gap here. We've travelled to UK together, and generally have the same likes and dislikes....my other two sisters....well i love them of course, but if they werent my sisters, i dont think they would be the kinda people i hang out with...so what im saying is, sometimes it does work, the larger age gap.....esp in later life.

ETA: This is based on a family with all girls.....i dont have any brothers. I dont know how a brother would have gone in this equation.....i.e what if your #3 was a girl with a 4-5 yr age gap...??? Something to think about....