1. No. Ages seem unlikely to be normally distributed. First of all, they are not normally distributed in the general population. The distribution depends on place and time, and there will be a significant correlation between children and their responsible adult.
2. No. Height may be more likely to be normally distributed, though again their will be significant correlation between young children and their responsible adult(s).
3. No. There will pretty much always be 16 glasses of milk in a gallon. There is no "distribution."
4. Yes. Gallons of milk sold in a week may very well be normally distributed. Among those who use milk, the range of variation is likely to be narrow, but adding a large number of users should create a nearly normal distribution.
Holidays or special events may cause some skew (or not, depending on demographics). Some may drink more milk when they have company coming; others may prefer wine or other non-milk beverage.