Calculate hat, waist, and jacket size when given age, height, and weight
I see some things that may help you improve your code.

Don't abuse using namespace std

Putting using namespace std at the top of every program is a bad habit that you'd do well to avoid.  An alternative that may still meet your needs may be to put it within main rather in the global context.

Eliminate parentheses to enhance readability

Adding parentheses where they are not needed makes the code harder to read and comprehend than it needs to be.  For example, in the hat function, we have this:

return ((weight/height) * 2.9);


With no change in the effective calculation, we can simply write this:

return weight / height * 2.9;


I think that's a little easier to read. 

Fix the formatting

It seems you may already know this, but your indentation and use of whitespace is not consistent.  It's less important which particular conventions you use than choosing one and applying it consistently.

Specify parameter names in function prototypes

With a function prototype like this:

double hat(double,double);


I can't tell which parameter is the height and which is the weight without looking at the implementation code.  Since function prototypes tend to become the documentation for your code's public interface, make it easier on the user of these functions by writing the prototype like this:

double hat(double weight, double height);


Or even like this to make the units unambiguous:

double hat(double weightInPounds, double heightInInches);


Don't pass parameters that are not needed

The waist routine does not need or use the height, so that parameter can and should be eliminated.

Use all required #includes

The code uses toupper() but doesn't include the corresponding header.  The code should have

#include &lt;cctype&gt;


Simplify expressions

The jacket and waist calculations are more complex than they need to be.  For example, for the waist routine, one could write this:

double waist(double weight, int age)
{
    double waist = weight / 5.7;
    if (age &gt; 28) {
        waist += (age - 28) / 2 * 0.1;
    }
    return waist;
}


Note that the calculation (age - 28) / 2 is done using integer math, so that subexpression will yield 0 for ages of 28 or 29 as per the specification.  Understanding implicit conversions and particularly numeric conversions will be very helpful to you as you write future programs.