Why can't variable names start with numbers?

I was working with a new C++ developer a while back when he asked the question: "Why can't variable names start with numbers?"

I couldn't come up with an answer except that some numbers can have text in them (123456L, 123456U) and that wouldn't be possible if the compilers were thinking everything with some amount of alpha characters was a variable name.

Was that the right answer? Are there any more reasons?

string 2BeOrNot2Be = "that is the question"; // Why won't this compile?

Solution 1:

Because then a string of digits would be a valid identifier as well as a valid number.

int 17 = 497;
int 42 = 6 * 9;
String 1111 = "Totally text";

Solution 2:

Well think about this:

int 2d = 42;
double a = 2d;

What is a? 2.0? or 42?

Hint, if you don't get it, d after a number means the number before it is a double literal