Generally rainbowfish should improve in colour as they mature, but when they are old they can get a bit tatty around the edges. Still old fish would be in the 5+ year range and around 4" mark albeit females smaller. Diet is important and should be varied including a lot of green matter. Their staple should not really be too high in protein and many people choose to use goldfish oriented flakes/pellets over tropical.
Generally with rainbows (and most fish) the males will be larger and significantly more colourful. On top of that if you have a mixed group, one male will assume dominance and become the most intensley coloured fish of the lot. Another common way to sex is to observe the two dorsal fins. In adult males the first dorsal fin will often extend and overlap into the second dorsal fin. In females there is usually no overlapping.