You can add a step to the online booking process for your guests to specify the ages of all the people in the reservation.
For instance, the classifications could look like this:
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12 years and under
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13 to 17 years old
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Adult (18 +)
The first step on your widget will be to select the total number of guests for the reservation, then to specify the number of guests for each classification. The sum must equal the total number of guests, otherwise it will not be possible to continue the online booking.

On your dashboard, this information will be displayed in the reservation window at the bottom left:
