I don't think so.

Let

Then giving maxs/mins at x=0,-2b/(3a)

And giving infl pts at x=-b/(3a)

which is half way inbetween.

Now even if we change that to

the inflection pt is the same (x that is)

while the max/min is the solution to

which gives and that average is once again -b/(3a)

So it is true for polynomials of degree 3.

Moving onto the next power...

Try giving mins at x=0 and 2 and a max at 1

while giving infl pts at

and those two inflection pts are not at .5 and 1.5, the midpoints of 0 and 1, and of 1 and 2.