Since this should not be an issue that is best left up to people’s individual feelings, levels of maturity and preferences, we need to make a rule.
calculate the difference in age of the girl’s parents, and the boy’s parents, and the girl’s grandparents and the boy’s grandparents. Then calculate the average of those 4 values.
To that, add 1 year for every 1 inch difference in the boy’s and girl’s heights.
To be machmir, average the boy’s and girl’s IQs. If it is an even number, add another year to the value gotten above. If it is an odd number, subtract a year.