You need to apply your formatting to the string, not to the return value of the print() function:

print("So, you're %r old, %r tall and %r heavy." % (
    age, height, weight))

Note the position of the ) closing parentheses. If it helps you understand the difference, assign the result of the formatting operation to a variable first:

output = "So, you're %r old, %r tall and %r heavy." % (age, height, weight)
print(output)

You may find it easier to use str.format(), or, if you can upgrade to Python 3.6 or newer, formatted string literals, aka f-strings.

Use f-strings if you just need to format something on the spot to print or create a string for other reasons, str.format() to store the template string for re-use and then interpolate values. Both make it easier to not get confused about where print() starts and ends and where the formatting takes place.

In both f-strings and str.format(), use !r after the field to get the repr() output, just like %r would:

print("So, you're {age!r} old, {height!r} tall and {weight!r} heavy.")

or with a template with positional slots:

template = "So, you're {!r} old, {!r} tall and {!r} heavy."
print(template.format(age, height, weight)