Shorter = easier to tack. Longer = easier to steer when hiking hard, and and in very tight air. Top sailors use 125 or 130 cm (including the unbent joint), but a beginner might be more comfortable with something shorter. Mine is 120 and I'm very happy with it. The next standard length down is around 107, and that's the absolute minimum you can get away with. Sailor age is not a factor, and sailor size sort of cancels out - if you're taller you hike farther out, but then your arms are longer, too.
The standard length is 48 inches or 4 feet however I find that a bit too cumbersome as it some times catches the sheet between tacks so I cut mine down to 47in. I sail against a world champion that have cut down to 46in. The other thing to consider is your height over 6 feet you will want all of the 48 has to helm on full out hiking mode but the shorter you are the less you will need, I'm 5'11 so I use a 47. Some world champions I sail with are 5'10.5 use a 46in. I know a gold medalist that has a 42. It's a combination of height, style and experience there is no real scale of formula just what works for you but I hope this can help you on your way!