>>What do you put in the Age field when you don't know the age?
Good question, Viv.
The answer is that we need a rule.
Assuming that it's a numeric column, I might use -1 to indicate that the age isn't known. Someone else might use some other rule.
Why wouldn't I use NULL?
If I'm the poor bloke who looks at this table without knowing the rules, if I see -1 I'm alerted to the fact that someone did that on purpose and it must mean something so I'd better find out what it is.
If I see a NULL, I'm never sure that some sloppy programmer just didn't populate the column properly, as has happened to me on numerous occasions.