At what age does someone become "old"?
Look at your testicles, once your scrotum is no longer wrinkled at rest (and at normal temperatures), but instead shiny and taut, that's when you know that you've reached old age.
Another trick you can do is to masturbate after 4-5 days of not masturbating, if you're sperm appears liquid-like (with the consistency of milk) and scarce in quantity (say about 1-2 teaspoons), then you are most likely old (assuming you don't have any serious nutritional deficiencies or physiological problems affecting your sperm production). Remember that when you are young and virile, your sperm is plentiful upon ejaculation and with the consistency of thick yogurt.
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