Ages 4-6: When a person with a vagina and a person with a penis have sex, sometimes a baby can be made. The baby will grow inside the uterus of the person with a vaginas body, and usually takes about 9 months to grow before it is born.
Ages 6-11: When a person with a vagina and a person with a penis have sex where the penis is inserted inside the vagina, sometimes a baby can be made. The baby is fully formed inside the uterus of the person with a vagina’s body, and usually takes about 9 months to grow before it is born.
Ages 11-14: When a person with a vagina and a person with a penis have sex, sometimes a baby can be made if the person with a vagina has started menstruating. The chance of a baby being made depends on if the people are using contraceptives or barriers, and if the sperm from the person with a penis makes its way inside of the vagina. The penis does not necessarily have to be inserted into the vagina to accomplish this. If the fetus gets carried to full term, meaning there are no complications with the pregnancy, the baby will grow inside the uterus of the person with a vaginas body for about 9 months before it is born.
Ages 14 and up: When a person with a vagina and a person with a penis have sex, sometimes a baby can be made if the person with a vagina has started menstruating. The chance of a baby being made depends on if the people are using contraceptives or barriers, and if the sperm from the person with a penis makes its way inside of the vagina. The penis does not necessarily have to be inserted into the vagina to accomplish this, if semen accidentally drips down into the vagina it can also cause pregnancy, and can occur at any time in their menstrual cycle, even when on their period. Some people choose to get pregnant via things like IVF, which introduces a sperm cell to an egg cell without using sex. If the fetus gets carried to full term, meaning there are no complications with the pregnancy, the baby will grow inside the uterus of the person with a vaginas body for about 9 months before it is born.
These are rather rough examples, and the exact details and wording could change depending on the exact situation (time I have, how the question was phrased, etc) but they work as a kind of ‘skeleton script’ for how I’d answer different age groups.