I am absolutely terrible at delaying gratification (ADHD side effect), but I have been very pleased to note that my 3 year-old does it routinely. When given a treat, he'll often have a nibble, and store the rest away for later. I suspect it's a fairly natural stage that kids go through, but it requires encouragement to become established for life. Similarly, kids will go through stages where they exhibit pretty negative traits, and parents should respond by discouraging those behaviors (don't hit your sister!).
I have a theory that it's impossible to prove anything, but I can't prove it.