When developing muscle what is more effective: number of reps or weight size?

or both? For example Doing lots of repetitions with little weight or few repetitions with heavy weights? Or it depends of the goal and even it depends of the age of the person? I said agge beause a person who is llets say 12 years old won't be given heavy weights or a r person above 55yrs old either even if both want to define muscle.
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What is the best heaviest weight for a person in theri 50`s to build a defined muscle? Like me. I dont want to become bulkier but have a nice feminine muscle definition, but I dont want to lift heavy heavy weights either and I will never do it either.
When developing muscle what is more effective: number of reps or weight size?
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